I get a grinding kind of sound sometimes coming from the back end of the car.mostly when under a slow increasing load.dose that make sence?Like on a slow take off at about 10mph I step on the gas just a little harder or start going up a hill at 40ish and give a little extra gas to keep my speed.sounds like the rearend.But its not doing it all the time.At 55 to 65 runs super!Could it be my rearend or just spark knock.It's not running at 100%.I started to put 93 octain gas in it.hope it helps.OH,it's a 5.7tpi in a 89iroc auto.Ideals at 1000 to 1200 not so steady.

 

Could be a rear wheel bearing or low gear oil in the differential. Have you checked the level?

 

checked it and added about an inch.well see if that dose it.

 

it's doing much better.still some noise when medium acceleration is given.but if I take my time or take off fast theres no grinding at all.My wife thinks it might be the muffler.