Grinding noise

My 98 S10 2 WD auto started making a grinding noise underneath somewhere (around the center of the truck I think). Starts at about walking speed and then continues. Doesn't seem to get louder with speed.
Sounds exactly like bad brakes when applied, but isn't. I actually took off the drums and eliminated that idea.
Any thoughts?

A/c compressor can make a similar noise- does it make noise with a/c on?

the noise is not in the front