Electronic noise puzzler

[Obertelli]

Mine is a UK model, so doesn't have a speed warning buzzer. In any case the noise was independent of speed. The car has only done 68k miles and there's no other symptoms to suggest a faulty alternator, and even if it was faulty I can't see why only one speaker would be affected.

For now, the fault has vanished as mysteriously as it appeared. But as there was a blocked drain tube causing a wet carpet under the left seat recently, and as on the single day the fault occurred that seat had a passenger, I am leaning towards the idea that the two facts may be connected. But the fault no longer appears even when that seat is occupied, so as it's impossible to diagnose and fix faults which aren't there I'm going to hope it was a one-off oddity and if it wasn't try and deal with it if it comes back.

Thanks very much for the replies and suggestions though.

[Benjamin]

If it has gone away, it will be difficult to track down. If it happens again, I would maybe try swapping left and right dash speakers to see if the sound follows the speaker or stays on the right. At very least it would rule out the speaker. Otherwise, since the PCM is off, it would have to be an induced hum from something else that has wires in the same harness as the right speaker. I'm guessing that you haven't changed anything in the harness and since it wasn't happening before recently, it's not just a "re-route the wire" problem, something is producing a hum that isn't normal.

Do you have a durametric tester? It allows you to turn on different electronic components (fans, etc) one by one and you could see if there is something that reproduces the sound.

Good luck, and please keep us posted.

[Obertelli]

Thanks Benjamin. It hasn't come back so far, but that durametric tester sounds like something to add to my wish list.

The only work I've done involving electrics so far was when I replaced the right door window regulator, and that was just a matter of unplugging and reconnecting various bits.