My 88 560SEC does this as well, if you brake softly all the way to a stop they squeak, if you brake firm they dont make any noise and come to a nice quiet faster stop, if you brake hard they grumble and like just make an odd growling noise that is not like anything ive heard on other cars, ive kicked in many an ABS system before and none of them sounded or felt like this growling noise i hear. my car is also equiped with new pads on old roters, the brand of pads im unsure about they were on the car when i purchased it recently