Wow, it really was the heat shield rattling.
 

Unbelievable the way the sound travels. It sounded like it was coming from the engine compartment.
My rattle was the heat shield on the 'over the axle' exhaust pipe. At first glance, that didn't seem to be
a problem. But, after a closer look, the band clamp, which was still in place, was in fact broken. After I removed it,
the rattle was more prevalent. Installed a worm gear clamp, and NO more rattle.

Thanks, to everyone who posted the possible causes of some rattles. This was mine.

HF is the best.

EJ

Hey EJ, first of all I just wanna say I’m stoked you figured out the source of all that frustration. I know this is an older post, but I’m a new hummer owner and new to the forums and reading this really saved me some serious headache. I have had my 2009 H3T 3.7 for about a year now with some obnoxious metallic vibration coming and going randomly around 2000-3000 rpms and just couldn’t figure it out. The sound seemed to move and come from different parts of the truck, depending on the day and the conditions. Some days it sounded like it was coming from the rear. Others the driver side front. Still others it sounded like it was behind the center of the dashboard. I’ve suspected everything from loose pulley wheels to rusted out exhaust clamps, you name it. I just read your thread and said wth, why not try and pull out the heat shield and just see what’s going on even though it seemed fine. Turns out, the damn thing was completely rusted out on the inside/underside that’s not visible from above or the side. It had basically split into 2 layers, with one an inside rust crumbling layer hanging a few mm or so off the original heat shield layer. Took a wrench to it, drove around the town in all the usual routes and RPM ranges, and whadya know, nothing. The sound is gone!

What a great community this is!

Like the needle in the haystack.

Find the rattle,

That is a new one ...