I think Mike's on the right track with either residual pressure, MC rod too long not letting the MC relieve, or heat affecting a brake line, one of which would be keeping your pads dragging just a bit. The key indicator is the excessive brake dust. I'd take it out for a drive, getting everything hot (hopefully making the mystery noise) then as soon as you pull in jack up each wheel and spin them manually to check for drag. You might also let it cool and see if the drag disappears which might point to heat from exhaust on a line or caliper? Just a thought....