The car is a 2000 2.8 z3. The audio system is basically original, HK, with a woofer in the center console between the seats. The problem is that the woofer in the center console is getting a speed sensor signal, so each rotation of a tire causes a "thump" signal to be transmitted into the woofer. This is not engine noise, but an electrical signal. If the electrical system & radio are on, and the car rolls, the woofer will "count" RPMs. Yes, it is bizarre. Naturally, as the car speeds up, this noise becomes a buzzing rumble, but at very low speeds the signal is obviously wheel rotation: thup-thup-thup-etc.. The first time this happened was after a large rainstorm caught the car un-garaged and some water (not a lot) got in the trunk. I suspected a transient ground issue; and indeed, when the car dried out, the problem went away. However, years later, the thup-sound in the woofer returned, independent of any new moisture. If I take the woofer amp in the trunk out of the circuit, the issue goes away ( I also then have no bass in the system, which is a bummer in the bimmer). But the issue is not the trunk amp unit; I have tried two different ones. Someone in another forum traced a similar problem to the speed sensor that goes to the radio (pin 10 / black-white wire in the head unit). I have that wire in the head unit harness, but my radio does not receive that signal and has no receptacle for pin 10 in the "in" harness to the radio. (I had high hopes for that possible fix, but that seems irrelevant.) I'd be grateful for any specific intelligence on this problem. It is rare, in my research.