Squeaky brakes

Why do the brakes on my 2003 Volvo S60 sedan squeak as I approach a stop at less than 10 MPH? There are 46,000 miles on the car. The front brake pads and rotors were replaced in Feb, 2007 at 33,000 miles. They were checked during the 45,000 mile service and found to be OK.

The pads vibrate like fingernails on a chalkboard. There are compounds that you can put on the backsides of the pads to reduce/eliminate the noise. Its going to be costly to put it on now, should have been done during the brake job.