I've got a strange grinding sound that keeps happening when I'm pressing the gas pedal lightly. It also happens when I'm not pressing the gas pedal. Usually happens at low speeds and sounds like it's coming from right underneath the driver's seat, but that of course could be wrong. The RPM's aren't fluctuating, and it doesn't affect the speed. What could this be? It's definitely in the drivetrain somewhere, but I don't know what would cause this sound.

 

If it sounds like it's under you, the tranny is right there between seats under the floor.

 

Check tranny fluid level (warmed up and in Neutral). Also there are some heat chields underneath tha could be vibrting at certain frequencies. Also look fo vacuum leaks...intake manifold. Does you heat/AC work?

 

Fluid level is perfect.

AC works great. I replaced my water pump and radiator a few months ago, and the heat hasn't blown the same since. I don't know if that would be vacuum related or not.

 

The transfer case is more under me than the transmission, but even so, wouldn't shifting be affected if something was grinding in the transmission?

 

Have you checked the transfer case fluid level? Check the front and rear drive shafts also. The cardan joint on the front shaft could be suspect.

 

When the bearings in my transfer case were going it was making a grinding noise like you describe. It eventually got to the point were it was constant. +1 on what Zman said on checking the fluid level in the transfer case and going from there.

 

I would also drop the rear shaft, and pull the tailhousing off the t-case, and see if the snap-ring hasn't abandoned the shaft.

 

I had the same problem in my 1996. Ends up that the transfer case had a HUGE leak (Probably cracked). I had driven it a LOOOONG time while it was bone dry. Even when you are not in 4x4, the transfer case still spins, it just doesn't engage.

Check the TC fluid level.

 

So did I, I didn't notice it was completely empty for the first two months I had my truck and who knows how long the previous owner didn't know. It also went in and out of 4wd with no noise and it didn't make any sounds.