1994 Explorer Screeching Noise from brake pedal?

My 1994 Explorer recently started making a strange variable screeching noise when I press on my brake pedal. The real strange thing is that depending on how hard I press on my brake pedal. The noise varies quite significantly. At any given pressure on the pedal, I can hold a steady tone. However if I pump the pedal it almost sounds like a ambulance siren. I recently replaced the front pads but this had no affect at all on the noise. Although it really does sound like the noise is coming from the brake pedal itself. I'm very sure that it is actually coming from just the other side of the fire wall. I should also tell you that my "ABS" light has been coming on intermittently for about a month or so. The new brake pads also had no affect on the ABS light either. At this point although the brake system seems to be working properly, I'm thinking that maybe it's a brake booster problem. However I'd really like to pin this problem down before I go any further. I am very mechanically inclined and should have no problem doing the work myself. I just want to be a sure as I possibly can be about what this problem could be.
Many thanks for any assistance and/or advice you may be able to provide.

Larry Bailey