Braking Noise, Even while stoped.

I have a 94 Buick Lesabre, when I press the brakes I can hear a sounds like rubber rubbing together, similar to Whooop-Whooop-Whooop. Sometimes it does it, sometimes it doesnt I cant pinpoint the cause but it doesnt matter if im parked or moving it can occur randomly. The Power Brake Light has also come on.

The only rubber part I know about the braking system is in the brake booster, if the diaphragm has a leak could it cause this sound? And if so, is there a test besides replacement?