Greetings! I've been a lurker, but now fully joining the fun. We bought a 2003 GS a few years ago and this car has pretty much been (major problem) free.
One issue that I thought the car had was sagging rear springs. I bought Monroe Quik-Struts and had a GM dealership replace them around 2 years ago. Last year around January 2017, on a vacation to South Texas, the car started making interesting rear noises. I had the car into the dealership this summer and they said the struts were bad. I had them inspect the rear bars/bushings etc..but all seemed to be ok on that front.
I replaced these struts with AC DELCO struts. The car continued to make the sounds. Dealership on last oil change noted NOTHING wrong/they hear nothing/see nothing unusual other than the struts possibly being bad. This winter, we find our car back in South Texas in warmer weather, and once again the interesting sounds return worse than ever.
To give some ideas. the sounds do not happen with bouncing the car. They seem to happen with wheel hop/torsion. Sometimes the sounds will sound like a low rumble of thunder and other times the sounds sound like a combination of nuts/bolts in a grinder/machine gun rat-a-tat-tat style for about 1 sec. The sounds seem to come primarily from the left rear. Tonight upon closer inspection, it seems the lower shock mount is no more than a finger above/and in between the tire and the spring. Is this the way it's supposed to be? I have searched for the right picture but I can not find a good picture. I'm tired of the games/feeling nuts. Is it possible that a wheel bearing could produce such crazy noises? Nobody has suggested this, but with as many of these W's on the road, this car CAN NOT be the only one with such an issue. I appreciate any help that someone can provide!!!!, and will gladly send a WalMart/Amazon gift card to the first person able to give me direction as the dealership seems out of the loop.