95 sunfire grinding noise

Hiya all. Have a 95 Sunfire 5 spd that all of a sudden makes a pulsing grinding noise only when no load is on engine. (rolling in neutral, or driving at a constant speed ; not speeding up or slowing down.) it doent matter what the engine speed is , the noise slows down as the car slows. It is not the brakes as they where done and the noise is still there.It does sound like its coming from the left side and you can hear it real well from inside the car. Any ideas? Ran the car on stands and it made no noise. Drive it and the noise is back.

Ply your backing plates next to your rotors back a little