I have noticed that I get a rhythmic rumbling sound at higher speeds, it starts at above 70mph, never did it before the rumbling is intermittent with a regular spaced interval between the rumbles and with increased speed the time interval between rumbles becomes less and the rumble gets more intense so that you can feel it in the floorboard. It is speed related not rpm because I can keep it in 5 gear and run up to 3200 rpm without the sound, so here is what I have checked so far. Also, you can not feel anything in the steering wheel.
1. Balanced the tires, no change.
2. Checked u joints and they are not hot after a run and I replaced them 10000 miles ago.

Suggestions as to the next step appreciated, thanks Jay.

 

i had a rumble that started@65and went away @75-80 replaced the rear drive shaft and felt like a new truck. cost me a little over 200 buck from the dealer for a compleat drive shaft

 

Your post is a little confusing. If it is speed related then it should not matter what gear you are in or what rpm you are spinning, but you stated that if it is in 5th gear the noise doesn't appear. To me, that sounds like you can rule out anything from the tranny backward. The rumble would be there because the driveshaft spins the same rpm at a given speed regardless of what gear you are in.

If you can't feel it in the steering wheel, then it is probably not front end related.

If it is only happening in 6th gear, then I'd be looking at the NV5600 for the problem.

Just in case I misunderstood - Does your rear driveshaft have a carrier bearing. Check that too.

 

I meant that it doesn't do it in 5 th to rule out the motor as a cause related to rpm because I can only get up to about 70 or so by about 3000rpm. What about the front rear bearings? What kind of symptoms do these cause when they go bad? It will do it even when I knocked it out of gear going down a mountain to maintain speed. I have got some more testing to do, I appreciate your all's input. I will let you know what else I find. Jay