Sounds like the rattle is coming from the right side rear. Almost sounds like a popping sound, or knocking sound. Give me some input on possibilties................................
 
 
I'm having the same noise, I just noticed it now that I've layed my back seat down. I thought it was the exhaust but thats tight as ever. What do you mean by Vent?
<img src="http://ojcsolutions.com/images/sig.jpg">
 
Check the strut mounts.  I had the same problem in my Cavy, my cousin had it in hers, a friend of mine had it in his, and in every one it was the strut mounts.  Apparently there was one of those GM bullitins or recalls or something about it.  I read it in a thread on htis site but I can't remember where.
Opfer benotigt. Keine Erfahrung notwendig.
  
Victims needed.  No experience necessary. 
 
 
Hmm.... my mounts were supposed to be replaced a while back but who knows if the guy I had do it actually did it right? Yea, and what do you mean the vent....???? Could it a possible wheel bearing going out?
 
 
Don't know.  When I replaced my struts it went away.  Same with my cousin and my friend.  But I think it's the actual mounts.  Loose bolts or something like that.  If you can find the thread, it should tell you.  Do a search and maybe you can find it.
Opfer benotigt. Keine Erfahrung notwendig.
  
Victims needed.  No experience necessary. 
 
 
If it's a clunky sort of rattle it's probably the mount. Could be loose, probably just bad. They're infamous for going bad, quickly sometimes too.