My HH pads squeal under certain conditions. Light braking under 5mph when rolling up to a stop light, has that high performance brake chirping. Or very hard braking as it dips below about 15 mph, this one sounds terrible almost like a big part on a lathe ringing.
Yeah sometimes the pads will pick up dirt. Careful application of a green pad can clean them out. Also if you bevel the leading edge of the pad it will help reduce the resonating factors that can cause a pad to vibrate at audio levels.
We "clean" our disk rotors after every endurance event. Rubber, dirt, and oils get tossed on the rotor from the tarmac and then contaminate it reducing performance. Simply "brush" them down with some 100 grit sand paper for optimum performance. I wouldn't use scotch brite pads. They will leave a residue that is as bad as the road debris you are trying to remove.