Knocking and Grinding Sound 2012 Saturn L Series

I have a 2012 L100 Saturn with just over 189K miles (yes, it’s been an awesome car). In October 2012 I both sets of breaks replaced plus rotors. About 2 months after the service, I started hearing a knocking and grinding sound coming from the bottom of the car, about 80% of the time when I apply the breaks. The car does not make the sound if idle or while accelerating. The knocking seems to be loud at times and less present other times. The grinding sounds like a large wire cable is being pulled right underneath the car. I took it back to the break place and they replaced the Master Cylinder under the warranty. The sound went away for about a month and then came back with about the same frequency. Took it back to the same place and this time they replaced the Wheel cylinder. Sound again went away, but this time came back 2 days later. I’m puzzled because for some reason the sound seems to go away and come back when the car is serviced. It;s also not consistant, but it’s there. Not sure what it could be related to if not the break job. I’m bracing myself for some really bad news as this car has been a life saver since I bought it. Any ideas?

hmm. master cylinder provides brake force to all 4 wheels. you have front rotor or disc brakes and rear drums which is where the wheel cylinder is. its very hard for most people to determine noise issues from brakes with certainty whether it is front or rear brakes. you implied that all 4 wheels had a recent brake job? you said “both” sets of brakes? you seem to be hopping around here. you mention noise and shop does this and that. do they really have any idea what is making sounds?