I put my 99 Silverado Z71 in reverse this morning to back out of my driveway and heard a buzzing noise, like the sound you here when your ABS brakes areactuating. I looked at the dash and my ABS light was on. I mashed the brakes a few times hoping it was just something sticking and would break loose but it didn't. I parked it and shut it off and the noise still remained. I got underneath it and saw a small motor on the driverside, inside the frame rail, that was running. I tapped it a few times with a wrench and it still wouldn't stop. I ended up unplugging the battery. Any ideas what I might be looking at? Anyone had this problem before?