It sounds as if needles are dropping, BUT as if there were thousands. I've heard this same exact noise on tons of vehicles. Its not a grinding noise, its a weird noise. About two weeks ago, my rear brakes started squeaking everytime I apply them. I had all my brakes inspected about 2 months ago and had the front rotors replaced because they were wraped. The back brakes were out of adjustment and the adjusters were missing. My E-brake is broken too. Does this sound like a bad wheel bearing? The weird noise just started today and its constant. When I stop, the noise stops. When I go, the noise starts up. I dont think anything is stuck or rubbing on the wheels.

its a 93 silverado z71 with 251,000 miles.

 

I had, for quite a long while, a sound that was similar to a stone being ground between the brake disc and the pad. I drove it for a couple of thousand miles like that without being able to isolate the source of the noise. It turned out, all the wheel bearings were ground to dust and the sealed unit had to be replaced. I was amazed it had not fallen off. The wheel felt tight when pulling sidewise on the tire and wheel. Good luck.