Strange noise help

[LA350]

Okay, When I am driving going about 30 through 35mph, 3-4th gear about 1500 rpm, I get this grumbling sound. If I stay in that speed range the noise gets fairly loud but when the RPMs go up or down it goes away.

I thought it was the clutch, but I am beginning to think otherwise. I wish I could post a sound clip. I was showing my son one night by slowing down and dropping to 4th and I couldn't reproduce it, until I pulled away from a light going up a slight hill then VROOOMMMGRRRRRRRRRVROOOOM

It is driving me nuts!

 

[Dieselguy123]

Input shaft or pilot bearing maybe? Not saying its your transmission but take a look here, if nothing else it is a good read.

 

[jtate]

has the rearend ever been ran low of oil, or have you ever checked it?