I have a 2004 h6 with 154,000 miles on her and she runs great except for one thing. At speeds of 25mph plus there is a low rumble sound coming from under the front seats. It starts at about 25 mph and will increase and decrease with acceleration and deceleration. I checked the wheel bearings and they seem to be fine. I checked the cv axles with no leaks at any of the boots. I just changed the trans fluid and filter. I checked the rear drive axle and there is some play when you turn it clockwise and counter-clockwise, but not up and down. I bought the vehicle with 133,000 on her so i don't know the history of the vehicle. How common is it for rear drive axles to be replaced on gen 2's? I'm not saying that is it but just a hunch. I am not a mechanic but I did stay at a holiday inn express last night J/K. Anybody have an idea what it might be or what else I could check to find the problem? Any help or info would be much appreciated.