Looking for a few ideas

Just bought a 91 Isuzu Rodeo LS -
Clutch, and a LOT of other parts were replaced less than a year ago... so we figured we could fix it up, but this has us stumped.
When going into first gear you can not hear the noise, but after reaching about 15mph you can hear an almost "roll of your toungue" sound coming from under the vehicle. It gets louder and louder as you speed up, into 2nd gear, all the way through 5th- making a ride on the highway almost unbearable due to the sound.
It is not a grinding sound, and it is constant reguardless of clutch position, you can even hear it with it in neutral and coasting.

Any ideas on what could be causing this? - And if you have an idea do you know how much it would cost to have it fixed?

Thanks!

Are you 100% sure that it's in the drivetrain?

Reason I'm asking, is when my 1st set of tires went out on my Rodeo, it sounded very similiar to the description you gave. There was nothing wrong with tread depth, still plenty of that left, but the tires had rotted from the inside out and they sounded horrible. The sound got worse with speed - took me weeks to narrow it down.

Probably not on the right track, but it is something to look at, especially if those tires have been on there for 3+yrs.

That is the first thing that came to mind was tires.