Electronic noise puzzler
 

Yesterday, and totally out of the blue, my right-hand drive 2002 2.7 Tiptronic S started to make an odd noise.

It's an electronic squeal/hum (difficult to describe) which lasts almost three seconds and repeats every five minutes without fail, and it comes solely from the right hand dashboard speaker.

The sound varies in volume going through cycles where it gets louder and then goes back to being very quiet before gradually getting louder again. It occurs with the PCM 1 stereo unit switched on or off and also occurs when my wife and I have our phones turned off completely (was hoping they were causing some kind of interference).

The sound occurred in daylight when the lights were off, at night with the lights on, and functions like the climate control and heated seats made no difference to it.

It's such an oddity that I'm at a loss to identify a cause, it's tempting to blame the PCM 1 but the fact that only one speaker is affected and that it happens when the unit is turned off suggests that's unlikely to be the cause.

The sound and the regularity of the timing suggest to me that it may be some odd capacitance issue whereby a cap is charging and when the charge reaches a fixed level after five minutes it discharges through that speaker. But that cap could be an electronic component or it could be something non-electronic acting as a cap and building a static charge perhaps due to something rubbing against it.

But that's just a bit of hypothesising and gets me no closer to an answer. What I'm really hoping is that someone out there has had the same fault before and managed to isolate the cause, or that someone with greater knowledge than I have can suggest further diagnostic tests to try and isolate the cause.

Here's hoping...

how's the alternator? has it been replaced recently?

Is there water in the car? You might check behind the seats. The comment about it happening with the radio on or off is similar to a problem I had in a Ford Mustang. The roof leaked, and the resulting humidity caused a similar problem with the radio off.

As you sure you did not accidentally set the speed warning buzzer on?

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.

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.

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.