I've got an usual problem. Just got my new Octavia 1.2 TSI SE (brand new, built 2011, facelift model). Love it, it's great! Standard spec, just one option fitted - rear parking sensors.

I discovered the other morning that if I have the ventilation on full (mark 4) on the windscreen, when I get out and close the door (as I had to scrape ice off the windscreen), the exterior of the car makes a solid warning tone?! Not too loud, but all I can figure out is it's coming from the back of the car somewhere. If I turn the ventilation down to 3 or lower, and close the door again, it doesn't make the noise.

I've got someone to check if it still makes the noise outside the car if I'm inside it with the ventilation on full, and indeed it does. So anyone walking past the car knows when I've got my ventilation on max!

The only thing I've noticed is, if you walk behind the car while the tone is sounding, it gets a bit quieter and then gets louder again as you reach the side of the car - which suggests it's something to do with the parking sensors.

But why a tone on the *outside* of the car? And it's not even in reverse, it's just idling in neutral, so why should it have anything to do with the parking sensors?

Is this a feature? Or some kind of genuine warning tone? Or a fault? Has anyone else had this before?!

Thanks!

OK, I'm *partially* being stupid. It doesn't seem to be linked to the parking sensors. The noise is coming from the rear driver's side door, and it's just that you can't hear it when you move round the back of the car.

What I think is happening is that something on that door is acting like a 'reed' and making a whistling noise due to the force of the air flow inside the car when the ventilation is on maximum - because I tried leaving a window open a crack before shutting the door, and it doesn't happen.

It's surprisingly loud, and it *really does* sound like a pure digital tone - like a warning signal or something. I guess the car is very airtight, and the airflow is finding its way out through that door?

Weird, because the car is whisper quiet inside, no wind noise or anything like that in the cabin. Strange problem?! Anyway, I'll live with it. I hardly ever put the blowers on max anyway, and even if I do, it'll only annoy people outside.

There is a one way valve at the rear to release the overpressure when you close the doors. If you listen carefully you can hear it moving with everything off when you close the last door (windows closed).

I suspect that what you are hearing is forced passage of air from your ventilation whistling through that flap but it is a wild guess.

Yes there will be a ventilation hole somewhere at the back of the car to allow air to escape when you have the fan on, if the air is being pulled in, it has to go somewhere doesn't it. I think you have hit the nail on the head with the reed theory. Air is being forced out of the car with enough force to vibrate this "reed" The parking sensors are not connected in any way to the ventilation system.

Ian

Thanks for the replies. I understand the reason now, but I presume it's not actually meant to make that noise. But like I say, I can live with it, I rarely put the ventilation on maximum anyway as it's pretty forceful!