Recently I've noticed that under heavier braking, after a certain threshold, there's a somewhat high-pitched wub-wub-wub sound that gets more pronounced the heavier the braking. It doesn't feel like there's a loss of braking power, though.

The car has about 42,000 miles on it, with original pads (A lot of highway miles without braking). According to the last oil change report, I have 6mm on the discs.

Considering this only happens under heavier braking, I'm thinking it's related to the drums, and I have driven with the parking brake on several times, but probably only a few hundred feet total.

So, any thoughts on the reason for the noise?