1997 Ford Ranger XLT
3.0L
5 speed manual transmission
4 wheel drive manual locking hubs

I recently replaced:
All front pads, rotors, and bearings
All rear shoes, drums, wheel cylinders
3 u joints on rear drive shaft and had the drive shaft balanced.
I had all 4 tires changed (having issues with them and had 3 changed already as they were defective, last time get tires from walmart)

Dont know if this sound was there before but dont think so.

1. ONLY when I hit about 75-80
2. ONLY when stepping on the gas pedal , sounds like a hum or a microwave
3. I let off the gas and it goes away.
4. Does not seem to increase in pitch or pulsate in tone....just gets a little bit louder...
5. If does NOT seem to be RPM based...as it does not happen in any other gear...only like 2800 rpm when doing 80...

hard to tell where coming from...but seems to be coming from under the seats, more towards the front of the truck than the rear.

Any thoughts on what I can check?

 

I have something similar to that going on in my 02. I doubt they are related though because you have hubs that unlock. If I take off my front driveshaft the noise goes away. I have narrowed it down to the transfer case front output shaft that spins all the time. I think it is the needle bearing on the front output shaft on the inside at the back of the transfer case. I think it is a little loose and get into a kind of resonance creating this humming in the 75 to 80 mph range. It goes away again at higher speeds.