Loud banging noise after car put in for service - what did they do?

I would really like some advice. My car went in for MOT and the handbrake needed fixing as it was insufficient in the rear offside. They changed the pads, disc and eventually caliper. After picking the car up I noticed a really awful banging sound, like a helicopter propeller. I took it back to the garage this morning and barely made the drive there as the sound got louder and eventually there was some very loud bangs just as I arrived. I left my car with them and they have repaired it (for free) but they have been very coy with revealing what the actual issue was. I would like to add that this is after consistently rude and poor service from them throughout. I am just curious if anyone can shed any light on what the issue might have been.