Prodrive Junkie

Just the other day I noticed a strange noise while cruising on the highway. It seems to start at random or when I hit some sort of bump, and it stops when I apply the brake. It's definitely coming from the passenger side.

It sounded to me like a brake shoe was dragging in the rear, or something in the drum had come loose somehow. I took off both passenger side wheels tonight, and opened up the rear drum, and visibly, I can't see anything out of the ordinary.

The noise is kind of high-pitched. Any thoughts? Thanks.