2007 H3 has a fairly loud rattling sound while accelerating from a stopped position. It sounds like a marble in a paint can, and starts slowly and speeds up with acceleration. Around 2500 RPMs or so, it stops. It doesn't happen while the vehicle is in park and i rev the engine, and it doesn't happen if in drive and i hold the brakes while revving. Anybody have any ideas?I've checked heat and skid plates, they all seam tight.

 

I know this is probably not going to be the answer to your problem, but i once had a saturn that had this same problem, I changed the coil packs and spark plugs and that did the trick. But, a Hummer is completly different than a Saturn.

 

Exhaust manifold heat shield, search it, they are known to get loose and rattle.