Warped rotor and worn out brake pads.
A grinding noise when braking can occur when the brake pads are going bad. You are hearing the brake grinding on the metal discs.
wheel bearing?
They could, yes.
Possibly the wheel bearing, if it makes a whiring noise when you accel. Or cv-axel, if it sounds like its grinding.
Brakes
Common causes of a bent rotor in a vehicle's braking system include hitting a curb or pothole, improper installation, or wear and tear over time. Symptoms of a bent rotor may include vibrations or pulsating sensations when braking, uneven wear on brake pads, and a squealing or grinding noise while braking.
Bearings, cv shaft, or brakes.
no oil
Brake linings worn? Wheel bearing?
i hate answers.com
Certain types of tires may cause a brief humming noise after your initial acceleration. The transmission may also cause a brief humming noise.
I'd try changing tires, worked for me.