My GMC 2500HD (2004.5 DMAX/Allison/4X4) seems to have developed a new noise that I am trying to diagnose. I first noticed it last weekend while driving on I-81 in Pennsylvania. As I slowed down from 70 mph to about 60 (I was not towing anything) I heard what sounded like "road noise" or "tire rumble", but it was something that had not been there earlier on the trip. When I speeded back up to 70 it went away, but came back again the next time I slowed down. The truck has about 11,000 miles on it and has always been very quiet (for a 4wd diesel) up to now. It doesn't seem to matter what the road surface is, so now I don't think it is tire noise. And it doesn't seem to change whether turning or going straight ahead, so I don't think it is wheel bearing noise either. It does seem to be a bit less pronounced when I ease off the throttle, but it is always there, up to about 60 mph or so. It's almost like some kind of driveline resonance or something like that. It is not real loud and if it had been that way when I bought the truck I might have thought that it was just normal heavy-duty truck driveline noise. I'm sure that's what the dealer will tell me if I go there. Anybody have any ideas what it could be, or how I could try to determine what it is ?

Thanks in advance :)

It is under warranty I wouldn't advise getting dirty over it, take it to the dealer and let them fix it while you enjoy the free coffee...

I'm thinking driveshaft. Had a problem kinda like that...only it started at 75 & got worse till 83-4 then went away. It was the driveshaft.