If it's a grinding noise in the rear, and it occurs with or without brakes being applied, you may have a bad half-shaft.

The joints at the ends (or the ones at the differential) tend to make a grinding sound when they're worn. To make a cursory check, take a crawl under the rear end of the vehicle and look for rips or tears in the rubber boots covering these joints. If you see any tears, or grease spread all over the end making the noise, you probably have a worn out joint, which is remedied only by replacement of the half-shaft (axle half). When a boot tears, it allows dirt and crud to get inside and gradually wear down the bearings with each turn of the axle.

I had one of mine replaced to the tune of about $240 parts and a couple of hours labor.

Gil