Whats causing a grrrrrrh sound when approaching a stop sign?

The calipers could be bindiung. Two things can happen;
The pistons won't retract because they have so much build up on them they cannot retreat into the caliper.
More likely is that a pin that the caliper floats on is ceased or sticking. Try taking off calipers and make sure they move easily in out out off of the mount. If not get some brake clean and fine emery paper and high temp grease to clean the pins and holes then grease then. Check that pins aren't bent when doing so.