Clunking noise
 

My '07 H3 has a loud clunking noise emanating from what seems like directly under the center console. Its not constant, sometimes only appears when I hit a bump and sometimes when the road is flat. Also, I'm not crazy, but it gets worse in heavy rain to the point that it becomes more like a constant vibration. I have not modified the drivetrain at all and the truck has ~63,500 miles. Oh yeah, and its a 5-speed.

Get underneath it and 'wiggle' things around. Could be practically anything. I had a '95 camaro years ago that made a similar noise. Ended up being the exhaust banging against the heat shield.

Thats exactly what I would do. I hear something loose, I am under my truck shaking and wiggleing until I find it. Then I tighten, lubricate,or replace the culprit, and end up driving for three days with the radio off to make sure the "ghost" didnt come back.... did I mention OCD.....

Good luck finding the noise!

oh yeah...can you "feel" it or just hear it?

If I rest my arm on the center console I can feel it. I've had passengers also tell me they can feel it in their feet. I dont feel it, however, if I grab the stick shift.

Constant vibration while at speed....First thing comes to mind is driveshaft. Shafts are not only machined from various metals for weight/engine load capacity, but also harmonically balanced for various RPM's. Hummer factory shafts are very durable, heavy, but durable. U-joints may be an issue that would cause drive shaft vibration. That clunking though, tells me that something is smacking, likely when you hit the gas, is my guess. (aside from bumps). If the U-joints were loose enough to 'clunk', brother you would have a lot more problems than what you do! Either way, wiggle-check the shaft and exhaust.