I got under the 07 H3 (84K miles) when I got home from business today and had a thorough look.
Nothing seems out of place. All items seem tight. I grabbed, tugged and pulled. As the title states this is an intermittent issues. I can be driving along and it will do the clunk, thump a handful of times and then for blocks it will be smooth and quiet.

The front diff bushing was replaced a couple years ago. All the shocks were replaced too. Lol! This H3 has had awesome maintenance as I'm a picky ***** for preventative measures. I use it for biz weekly so it has to run....

The Back drive shaft was rebuilt at the beginning of the spring and all the brakes are maintained and working.

Are there any known issues with Hummer H3 trans mounts or torsion spring mounts failing or such???

 

Did you check front drive shaft?

 

I'll be rechecking the rear shaft so I might as well pull the front and check it to this weekend. Good suggestion.

 

For consideration: I'm waiting for my front CV joint to come in so I can replace mine. When I was checking mine, all seemed good until I shifted my TC into neutral. It was only then (with the tension off the drive shaft), that I could verify it was bad.

 

Well I came right home to the garage today and got started. Took the rear drive shaft off and checked again. Greased the splines and the outer off the spline housing. Pulled the front drive shaft out completely. Other than the boot having a bit of mild dry rot there really wasn't anything. Of course there is one thing that kind of made me go "hmm?" The CV end felt kind of tight at its most flexed position when swirling it around. I'm not certain if that is normal. Changed the T-case fluid and took it for a test ride.

I'm wondering if that CV should just be replaced this next week?

 

Try and pump in some fresh NLGI #2 Grade GC-LB Lithium Complex EP Grease with a Grease Injector Needle

 

Hey Happy Hummer, I had a similar issue with my H3 when I bought it. I am unsure if this is the issue you are experiencing, but the front axle support bushing is extremely prone to failure and leads to a clunking noise like you describe. Here's a video on replacement, which shows the issue in further detail.

 

That's already covered back in first post lol but hey ....

 

Oh shoot, sorry! Maybe it went bad again already? They are truly terrible quality haha. It's really easy to check with just removing the lower skid plate

 

Installed new driveshaft after biz 2day. Seemed nice.

Just in case I checked it again 2day when under there....Dylan R.