I have started to hear a horrible sound coming from the engine. The car is a 2000 Jetta VR6 5 speed with about 49k miles. I hear and feel a terrible knock at idle. The idle had always had a kind of cadence to it, and the knock is regular with this sound. I have not yet changed the spark plugs, due at 40k. I cannot imagine that such a horrible sound would be caused by this, not even 25% overdue. I have heard something about a recall on ignition coils, but I think it was only for the '01/'02 cars. Any ideas?