It started out as a squeak when I went over bumps, but now its squeaking almost once every two seconds even as I'm driving on flat ground. I think something is coming loose in there but I'm not sure what. This is on my driver's side, I'm gonna take a look at it when I get a chance to take my rear wheel off, but any idea what it might be?

The squeaking sorta sounds like...a badly oiled shopping cart wheel, but loud enough that it can be heard in the interior while music is playing.