I've noticed while driving that the passenger seat shakes a good bit at idle and while driving. If I hit a bump, it shakes like a buzzer went off & moves around and shakes just a little bit. The rear captain's chairs don't do this & I assume the rear 3rd row doesn't either. They are solid. It's just the two seats up front.

I'm in the process of replacing almost all of the steering components & upper control arms (everything is stock right now & worn out). I know this contributes to the vehicle shaking, but the rears still don't, so I wonder if it's simply related to the seats themselves.

I'd just like to know if anyone's come across this so I don't waste time trying to diagnose issues that aren't related.

 

I would think it's a low frequency vibe coming thru the floor and being trasmitted to the seat. What's under there, transfer case? Or maybe the front drive shaft missing its balance weight?
My recently-acquired '04 has 140k and has no shakes, but the front suspension IS shot. Will be replacing uppers and lowers plus whatever else is worn out.

 

I think the transfer case does sit beneath the passenger or there about.
I'll have to investigate what's holding it up after I get the front suspension sorted out.
A lot of the shaking I was getting smoothed out when I upgraded the bushing on my differential cross memeber. It didn't entirely go away, but it helped, at least for a period of time. I should make sure it's still torqued tightly enough.

Nice catch.

 

My 05 front passenger seat rattles too when hitting a bump and so does the headrest. I'm sure it's just the seat. I know the headrest had a metal support down in middle of seat which is probably broken or worn out.

 

Welcome to the Hummer Shake and rattling world
Mine does that since I got it 60k miles .. still does but I don't pay attention to it anymore ...
Just the way H2 is

 

My 03 never did at 120K+ miles. The 08/09s don't have the issues involved with the trim shaking. It's entirely different, but this truck was driven hard, so that's the cause.

I'll first look into what mcioci said. I'll bust out the service manual & see where all the screw locations are. After that, I'll move onto other things.