So I have nooo idea what this problem could be. I just recently got aftermarket rotors and brake pads installed and went back to my stock s4 wheels. I'm pretty sure that it isn't the problem, but basically I was driving around and noticed a loud "knocking" sound at all speeds. I first heard it on the freeway but it was only when I would let off the gas it would start making that sound. Whenever I'd get on the gas or whenever I'd break, it would stop. When I am at a dead stop, no knocking either. I have no idea where to even start looking but has anyone had this problem before?