I have a '93 Park Ave Ultra. Just recently I'm starting to hear a very slight squealing sound that gets louder the longer I drive the car during the course of the day. I hear this sound coming from the rear tires. The sound is more pronounced when I slow down to be barely moving and when slowing to a slow turn around a corner. I was wondering if this sound is a rear brake indicator informing me that new brake pads are needed or is it something more serious. Thank you for any help you can give.