beltsander TKO

this just started happening on friday... i've noticed that whenever i step on the brakes, and about under 15-20 MPH, i hear this awful click. usually once. brakes seem to work fine. nothing is wobbly. the car doesn't seem to pull to either side when stepping on the brakes. don't know what this click is. it sounds similar to when a cv-joint in a fwd vehicle is about to fail. except this only happens once. the sounds appears to be coming from the front of the vehicle, on the driver's side. this click also happens when i have the car in reverse, and applying acceleration.

it's snowing outside, so i couldn't take off the wheels to check... any ideas?

thanks!