Can You Guys Help Me?

[CO Hummer]

I had two muffler problems, and one was exactly what you are talking about. It made deep vibrating/rattling sounds particularily when going slow with some torque applied. The easiest way for me to replicate it for the dealer was to slightly press the brake while slightly pressing the accelerater (while in Drive).

I can't take a photo for you but I can give the explanation I received from the dealer. They said that it was some sort of plate over (on top) of the muffler. Apparently this "plate" is very close to some other metal (maybe the top of the muffler itself) and that if it gets slightly out of aligmnent/loose then it vibrates against the muffler). They said the design is such that there is a very small amount of space when mounted correctly so a slight offset of the plate will cause it to rattle against the muffler.

The fix is quick as it only took them a few minutes to adjust it. So, I suspect you could do it yourself. My belief is that it happened when they were fixing a separate 'pinging' problem in my muffler. The tolerances (distances) of the moutings are so tight that when they reinsalled by muffler they just messed up the placement by a couple of mm.