could be a strut, maybe a bushing squeaking? the only way to tell is to get under there and find it.
My suspension squeak was caused by the front struts being shot(35k) and the two Strut Link supports being worn(rubber was rotted and torn). Next day shipping from rock auto on the struts and autozone for the strut links and car handles like a dream and no more squeak!
Sucks I had to blow money on the ride because im in the red more so than i was before :|. I was able to hand compress the struts with no resistance. Makes me think the rears may be due as well.
another common problem with gm cars is the sway bar bushing rubs creating the squeak, i have this problem with my g6. Not a problem, just a little annoying, so make sure you know what it is before you go dumping cash into struts