Not, it's not what I thought was the obvious answer.....

'02 F150 S'Crew 4x4; 4 wheel disks, 72k miles. Ford parts numbers on the brake pads; whether they're original or replacements I don't know.

In a nutshell, I get a continuous growl out of the brakes for the duration of time that I'm on the pedal. It sounds as though it's contained to the rear brakes.

When I started hearing it, I priced out new pads; because it sure sounded like the old "pads are worn, the rivets are grinding" noise from the old drum brakes days. But I've pulled the calipers on all four corners; the pads are bonded (no rivets to grind), and there's a good 5/16-3/8" of material on the pads. They're semi-metallinc pads; having distinct copper flecks in the surface of the friction material itself. Additionally, there are no "shadows" indicating runout on the rotors themselves.
Any ideas? This doesn't sound anything like the anti-lock system sounds when it kicks in on my '00 F150 4x2 work truck. Just keeps driving me crazy searching for the reason for the noise.

Thanks in advance for the help