I've got the same problem. Of course when I took the car to the dealer last week, we couldn't reproduce the sound. It's very sporadic; it doesn't make the noise when it's wet out (it rained the day I went to the dealer). It usually sounds like it comes from the rear. Sometimes it comes from the front. My guess is, it's coming from the springs in the rear and/or the strut mounts in the front. I've got about 21k miles right now and have noticed it for about 5k.
Trying to relate to and diagnosing sounds when we do not even know if it is front or rear is very challenging. I will offer some suggestins, but this does not mean your GTO is involved with them and we will assume everything looks normal:
1. Strut bushings can be collapsed and transmitting noises
2. There is a bulletin on spring contact to the lower seats. With this issue, there are witness marks on the lower part of the springs.
3. Leaking radius rod bushes.
4. sheet metal creeking
5. Anything loose
6. Mis-aligned end links (twisted)
1. Shocks and or mountinhg bushings
2. badly out of aligned rear cradle assembly putting a bind on the Xmember bushings
3. A wheel hub can generate all kinds of different noises
4. anything that is loose
5. Exhaust hanger problems