Engine: B8444S: Name that noise #2

[Moose Test]

Hi,

With the ignition in position II but the engine stopped, I hear quite a loud, high frequency noise within the engine bay.

To me, it sounds like a very faint nearly dead siren or horn running off a very flat battery (think 'olden days' run down car alarm) but punctuated by a random 'ticking', a bit like a Geiger counter makes. It seems to be coming from under the inlet manifold on the top of the engine, or in close proximity, and is a little louder on the passenger side.

Q1. Does anyone with a V8 (or other petrol model) notice the same thing? i.e. is this normal?
Q2. What might be the source?

Possible candidates that I can think of are a) coil whine from one or more of the ignition coil modules or b) it's the sound of high pressure fuel circulating in the common rail.

TIA

[oragex]

Have you listened with the hood open? May be the brake booster pump running which I'd say may meant the brake booster started leaking at the seal near the pedal

[Moose Test]

Yes, you can only hear it with the hood open.

Where would I find the brake booster pump? I'm finding it difficult to completely localise the source but to my ears, it is coming from under the inlet manifold on the top of the engine itself, which I would think eliminates most ancillaries?

[Tannaton]

Could be a valve in the vacuum system. Again not sure about the V8 but on the D5 there are vacuum valves that control pressure applied to the engine mounts, turbocharger VVT system etc. The pressure is controlled through PWM (duty cycle) and these can "hum".

I'm not sure where the vacuum source is on the V8 - it may be from the manifold or a electric/mechanical pump.

[Stu V8 2007]

My 2008 V8 has exactly the same noise so I guess it's normal.

[Kwozzymodo]

It’s the brake booster generating a vacuum on start up.

[Moose Test]

Thanks Stu, that's good to know.

[oragex]

If it's the brake booster vacuum pump, it's not normal for it to run when the key in put in Pos II before driving the car. It means the brake booster is loosing it's vacuum overnight. The booster leaks at the seal near the pedal, and causes the pump to run to compensate. In the long term the pump wears off and as it does so it becomes noisier.


https://www.youtube.com/watch?v=7m9Zwg7IbBw