We have an 08 LMM here that is making a light rumbling noise when going at regular road speeds. It sounds almost like its got some sort of mud tires when all it has are the regular stock tires. It does look like there is a leveling kit for the front that is on it. I checked the front differential fluid level and it appears to be fine, also checked the front wheel bearings an there is no play in them. I jacked up the rear wheels and had them spinning but couldn't hear any of the noise which must mean the noise is not coming from the driveline. So it's something with the front end but can't figure out what, any ideas??

tcase maybe?

can you feel it in the steering wheel? like something barely clunking but still got a tight steering??

Tires chopped?

@08dmax:
I'm experiencing something like that. Know what it is?

inside the intermediate steering shaft there are some type of knuckles, instead of GM fixing that problem they decided to go ahead and do a cheaper route and put a grease packing in it. it lasts about 2,000 miles if that. but the motor heat in the summer is so bad that it melts out the grease packing and it just goes right back to rumbling, or thumping or whatever you call it. they do have a fix everyone tells me but i havent did much research yet on it. i just have alot of buddies that have had the same problem including me with this issue and i know how to temp. fix it. i brought mine to the dealership and they knew exactly what it was. why they dont have a replacement fix for it???? no one knows. but i know somewhat what it is, and know alot of people with the same problems since the duramax came out.

STICKY: Steering Clamp Fix= your fix zack 280 ...look at the first thread on this forum...is the fix to it.

The tires must have been out of balance. Because it was fine after the tires were replaced.

If its not a tire issue it could be a wheel bearing. Just jack it up on the a-arm and see if the top of the tire moves. Keep in mind when the bearing are out it will affect wheel noise as the camber and toe will be jacked up. Should also be easy to look at the upper and lower ball joints at that time. Those could also contribute to that problem....The steering can cause that as well theoretically, but usually on my truck at least it will drift first or show other steering problems before it gets that bad....