I had the same problem, so I dynomatted the trunk and parts of the rear hatch. I fixed the rattling a little but mostly made the inside of my car quieter.

 

what the heck is dynomatting?

 

When the car is warming up or when the revs are around 1500rpm there's a rattling in my piping. I've thought it was the header or the exhaust canister rubbing on the rear bumper. I trimmed the rear bumper a bit and some of the sound has gone away. I might check the heat shield the next time I'm underneath the car.

 

Do you have an aftermarket muffler with a silencer? Sometimes the silencers come loose and they rattle....

 

nope my cars got stock exhaust.

 

mine does this too, but only at a low rpm (aprox. 1000) and usually only going up a hill of some sort... it doesn't really bother me though since.. it's only on those occasions

 

check the two bolts holding the cat to the header. u might want to either replace those two bolts or just lube it up with high heat resistant silicone grease and then tighten it up, but not too tight. that was the problem with my exhaust with it was stock. i thought it was the cat but then the dealership told me it was those two bolts.

 

if you have an adjustable angle front strut bar the mounts might be loose. mine was making that sound when i installed my front strut bar and i forgot to tighten the swivel bolts.