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Found this writeup and thought useful to post:

"Diagnosing Differential and Driveline Noises

We don’t know which sounds worse—us singing show tunes in the shower or a howling differential. While there is no help for our singing, we can help you diagnose the sounds coming from a differential gone bad.

A clunk, whine, or howl can signal a worn-out pinion gear, bad bearings, or a faulty gear installation. Sometimes, the noise is not differential-related, but is caused by other driveline or axle components. The guide below will help you match up the noise you’re hearing with the differential or axle component making it—or at least get you in the ballpark.

Noise: Howl while decelerating (gears previously quiet)
Cause: Loose pinion bearing preload

Noise: Howl with whir or rumble while accelerating at any speed (gears previously quiet)
Cause: Worn rear pinion bearing or worn gear set

Noise: Howl without whir or rumble while accelerating at any speed (gears previously quiet)
Cause: Worn gear set due to lack of lubrication or overloading

Noise: Howling while accelerating over a small speed range (gears previously quiet)
Cause: Worn gear set due to lack of lubrication or overloading


Noise: Howling after gear set installation
Cause: Faulty gears or improper installation

Noise: Low-pitch rumble at all speeds over 20 miles per hour
Cause: Worn carrier bearings

Noise: Whirring during acceleration/deceleration at about 10 miles per hour
Cause: Worn pinion bearings

Noise: Banging or clunking while making a turn, backing up, or rapid deceleration (like on a freeway off-ramp). Noise gets worse in warmer weather
Cause: Posi chatter due to improper lubrication; worn clutches or spider gears; improper assembly


Noise: Banging, crunching, or popping while making a turn. Noise not affected by temperature
Cause: Badly worn or broken spider gears

Noise: Banging or heavy clicking every 2-3 feet during acceleration and deceleration
Cause: Damaged or broken pinion gear tooth or teeth


Noise: Banging or heavy clicking every 2-3 feet during acceleration or deceleration, but not both
Cause: High spot or heavy chip on pinion gear tooth

Noise: Banging or heavy clicking every 8 feet during acceleration and deceleration
Cause: Damaged or broken ring gear tooth or teeth

Noise: Banging or heavy clicking every 8 feet during acceleration or deceleration, but not both
Cause: High spot or heavy chip on ring gear tooth

Noise: Clicking while decelerating from 20 miles per hour to a complete stop
Cause: Worn carrier case-side gear bores

Noise: Rumble or clicking that gets worse during hard turns
Cause: Bad wheel bearings

Noise: Driveline squeaking or grinding at any speed
Cause: Worn or damaged U-joints

Noise: Clunking when depressing the throttle pedal (takeoff)
Cause: Worn U-joints; worn spider gears; worn axle splines; excessive gear backlash; loose yoke splines; worn slip yoke splines

Noise: Clunk immediately after taking off from a stop
Cause: Worn slip yoke splines

Noise: Steady vibration that increases with speed
Cause: Worn U-joint or out-of-balance driveshaft

Noise: Cyclic vibration that varies in intensity. Intensity increases at a specific speed range, worsens during deceleration
Cause: Pinion angle too low (not parallel with front yoke on driveshaft)"
 

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Murphy the RMGTO Mod Dog
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Mine clunks everytime when you roll back into the throttle after coasting a few seconds or more at 70-80mph. Clunk is worse if you are coasting down or up hill. It also does it after down shifting from 6th to 5th or 5th to 4th. It is a dull, thud/clunk from the rear of the car.

So far the dealer is claiming they can't do anything because they can't diagnos it, because they can't open up the diff, being it is a non servicable part.
 

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Hendo Goat
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I was going to start a new thread but this seems like a good place to ask about the problem I'm having.
3 weeks ago I changed out my diff fluid because at 32k I was still running the stock fluid and was just beginning to get a slight whine at high speeds, I replaced it with Mobil1 75-140. Everything was great until about a week ago when I started getting a little clunk from the rear end when engaging the clutch in reverse, this has progressively gotten more noticeable and today I'm getting some clunking at low speeds such as rolling through parking lots, etc... I'm thinking it's fluid related because today is the warmest it has been here so far and it seems to get worse as the temp goes up. I'm hoping that's all it is because I'm out of warranty and really don't want to have to replace the rearend right now.

Is there a chance of doing any damage to the diff with this going on? Should I try not to drive it until I can get the fluid changed out to something other than the Mobil1?
 

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Murphy the RMGTO Mod Dog
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Mach5288, did you put a friction modifier in with the new fluid? If not, that may be your issue.
 

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I was going to start a new thread but this seems like a good place to ask about the problem I'm having.
3 weeks ago I changed out my diff fluid because at 32k I was still running the stock fluid and was just beginning to get a slight whine at high speeds, I replaced it with Mobil1 75-140. Everything was great until about a week ago when I started getting a little clunk from the rear end when engaging the clutch in reverse, this has progressively gotten more noticeable and today I'm getting some clunking at low speeds such as rolling through parking lots, etc... I'm thinking it's fluid related because today is the warmest it has been here so far and it seems to get worse as the temp goes up. I'm hoping that's all it is because I'm out of warranty and really don't want to have to replace the rearend right now.

Is there a chance of doing any damage to the diff with this going on? Should I try not to drive it until I can get the fluid changed out to something other than the Mobil1?
If you did remember the friction modifier, than it may just be the Mobil 1. I tried both Amsoil and Redline in my differential and I noticed that it was quietest by far with the OEM stuff - which is what I went back to and it is still completely silent. I also noticed that the OEM stuff is like 30 bucks a quart; I don't know if it's because it's GM or if it's because of a superior additive package. The OEM gear oil is fully synthetic and made from 100% group 4 basestocks (PAO - PolyAlphaOlefin - Esters), so it's a lot better than you would think. Perhaps GM really does know what's best for this one. If I'm not mistaken, that guy JET had the same experience with the aftermarket vs. OEM gear oils. One thing is for sure, the GM friction modifier worked WAY better than the Amsoil or Redline FM's (yes I changed my gear oil like 6-7 times before I found the best combo).
 

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Welcome to the Dark Side
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439 Posts
I have a clunking when slowly rolling away from a dead stop..I mean, extremely slow. It almost feels like the trans shifts (A4) and then the rear end catches up a second later. If I roll away with normal throttle or aggressive, nothing.

What do you think it is?

Also, has anyone experienced a noise almost like birds chirping? I thought it was my breaks, then I changed the diff fluid and it went away for a day..now it's back..
 

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Hendo Goat
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Thanks for the replies guys, I DID NOT use any friction modifier with the M1 fluid, it is fully synthetic and says it is formulated with FM for use in LSDs, like Royal Purple I suppose. Should I try to add some factory FM to the M1 fluid or completely change out the fluid again for stock and see what happens?

StreetSleepR, that is basically the kind of clunking I have as well, allthough I have a 6 speed, when I engage the clutch I can feel it grab but the rear end seems to catch and clunk a split second later, and it will clunk around at slow speeds like idling through a parking lot etc...
 

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Murphy the RMGTO Mod Dog
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Thanks for the replies guys, I DID NOT use any friction modifier with the M1 fluid, it is fully synthetic and says it is formulated with FM for use in LSDs, like Royal Purple I suppose. Should I try to add some factory FM to the M1 fluid or completely change out the fluid again for stock and see what happens?
Just pick up some friction modifier to add to the Mobile 1. I threw in Mobile 1 without the FM and it made noises almost immediately. Just drove to the parts store and got FM. Issue went away after that.
 

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Oh it's Friday night
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I think you need FM with Mobile 1, I have never heard that it has FM in it already. Even the factory synthetic requires FM. Mobile 1 and GM FM seems to work for me.
 

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Hendo Goat
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301 Posts
Well I picked up some generic FM at Napa over the weekend because the dealer didn't have any stock fluid and said they couldn't order just one bottle for me, I'd have to buy a case of 12.... Anyway, the clunking has not gotten any better, I'm almost 100% positive it's fluid related because I was not having problems before the fluid change. I guess my differential just doesn't like the M1, should I order some stock fluids and see if that takes care of it, and where is the best place to order them, because the local dealer obviously isn't very helpful when it comes to the GTO specific issues.
 

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METAL!!!
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To get the 7098 fluid recommended by GM for the GTO it is best to order it from GMpartsdirect. I just switched my car over to Mobil 1 after Torco. It was quieter than stock with Torco, but Mobil 1 made mine almost whisper quiet with 1oz of 7098.
 

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I Used Several Fluids Rp And Lucas With Poor Results. I Then Used The Thicker 75w140 Rp With Some Gm Fm And It's Much Better Now. I Think I Need A Lil More Though...

Amsoil Fm Did Not Work And Neither Did Torco Fm. These Fric. Mods. Made The Diff Too Tight Or Loose.
 

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METAL!!!
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I Used Several Fluids Rp And Lucas With Poor Results. I Then Used The Thicker 75w140 Rp With Some Gm Fm And It's Much Better Now. I Think I Need A Lil More Though...

Amsoil Fm Did Not Work And Neither Did Torco Fm. These Fric. Mods. Made The Diff Too Tight Or Loose.
I noticed mine was a bit loose with the Torco. When you used a different grade, did you end up using the 90 instead of the 140?
 

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Noise: Howl while decelerating (gears previously quiet)
Cause: Loose pinion bearing preload
I am definitely experiencing this, and I'm sure others have. The dealer will probably be 'unable to reproduce' it, but it's worth a shot. Thanks for the info!
 

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WANTED ALIVE!!!!
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I got one of the last two. i get a vibration thru my clutch pedal then it'll go away. I just put 700 miles on the new clutch and slave cylinder. I'm thinking it may be my driveshaft. what do u guys think?
 

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Rear end clunk - only when warm?

OK, I have question along this topic... I have an '04 with a little over 70K on the clock. When the goat is cold, there are no odd sounds from the rear - but when it is hot (30 miles or so of driving) I get clunks at low speeds, only when the clutch is engaged, and a weird growl during slow turns, again only when the clutch is engaged. I am gonna change out the diff fluid, but I am thinking this could be something bad...
 

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9 Second Club
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That is a pretty comprehensive list and most likely should be in the KB. I've been through a lot of this though mostly with performance parts. The clunking sure seems like it is from the LSD. Every new differential I put in I spend the 75 for the LSD spring kit which is money well spent.

I have tried several of the oils mentioned here with different results than some of the posted experience. In general I have been very satisfied with Torco RGO 85-145 and Ford FM. The synthetics seemed to break down and lead to more howling noise.

I am firmly convinced that twice a year the following should be checked:

  1. Torque of pinion flange bolts and Driveshaft
  2. Torque of central carrirer bearing
  3. joint just behind carrier bearing between two DS to look for wear or signs of damage
  4. Front Yoke bolts to driveshaft
  5. rubber donuts for wear and cracking
  6. mounting bolts for differential to body mounting
  7. Hex bolts mounting CVS to stubs and hubs (24 of them)
  8. Subframe connector bolts
  9. shock mounts
  10. swaybar bolts and mounts and bushings
  11. Front struts for rub
  12. front suspension fo leaks, wear, bent components
  13. Transimssion fluid levels
  14. Differential fluid leves (I flush and refiull mine twice a year.
That is just the way I do it, had to break a lot of chit to get this anal about it.
 

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I just went to advanced and go Mobil 1 75-145 for the rear and asked the manager if it needed friction modifier and he said no. From what I am reading here, I should put friction mod in. Where is the best place to locate FM if not from the dealer. I seen someone said NAPA??
 

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Oh it's Friday night
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Some people use Trans-X which is available from most auto chain stores.
 

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9 Second Club
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I just went to advanced and go Mobil 1 75-145 for the rear and asked the manager if it needed friction modifier and he said no. From what I am reading here, I should put friction mod in. Where is the best place to locate FM if not from the dealer. I seen someone said NAPA??
I can just about get Torco shipped to me by calling them faster than driving to the local autoparts store, especially during the week when I need to often work late. You can also get FM from them.
 
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