I have a 2001 Audi TT. While driving appx 70 mph, the car started making a LOUD banging noise coming from the engine compartment. When I slowed down the noise stopped. When I accelerated past 70, the banging noise came back, so I drove the rest of the way home at a reduced speed. The next day I drove to the store and about halfway there an ignition coil went bad. I recently replaced all four coils. Does anyone have a clue what might be going on?