# Slight hissing noise



## relic222 (Aug 24, 2012)

Okay, I don't really know how to explain this but there is a constant slight hissing noise that sounds like a cross between air escaping or the slight whirring of a wheel spinning. That's about as close as I can describe. It's coming from the front of the car and I can hear it when the car is moving or stationary. Oddly the noise stops if I put my foot on the brake, again I stress that I can hear it even when the car is stationary so it's not brake discs or wheels rubbing or anything. I know I haven't been all that descriptive but I don't really know how to explain it! Any help would be greatly appreciated.


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## YoungOldUn (Apr 12, 2011)

Leaking brake servo?


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## relic222 (Aug 24, 2012)

That sounds like it could be dangerous! It's been happening for over 2 months but doesn't seem to have affected anything noticeably. Is there any way to test if this is the issue?


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## sico (Feb 6, 2003)

Yeah check its not a leak from the brake system. Could be a servo or pipe.

Brakes would feel spongy though.


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## brittan (May 18, 2007)

The leak would be in the air admittance valve in the rear of the servo unit. The servo will still work as long as the vacuum valve part of the arrangement continues to function correctly and there will be no different feel to the brakes.
However if I'm correct you will have no reserve of servo assistance after the engine is switched off.

Some thoughts:
1. Is the hissing noise only there with the engine running?
2. Does the hissing noise stop on engine stop or continue for a short time?
3. The origin of the noise will be where the push rod from the brake pedal enters the master cylinder/servo unit. Get your ear down by the pedals and see if the hissing noise is more apparent.
4. About an hour after using the car and with the engine off try pressing the brake pedal. You should hear a slight hiss as you press it and this should be repeated for 3-6 presses until the vacuum in the servo is exhausted, when the 'feel' to the pedal will change. 
5. If you know what you're doing disconnect and blank the vacuum hose to the servo. Check if the hissing noise is still present. Do this check only with the car stationary. Do not drive the car as you will have no servo assistance to the brakes. Remember to reconnect the vacuum hose.


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## sico (Feb 6, 2003)

If the servo was damaged itself (casing) could it not hiss? Only say the above as I know someone who had a car where this happened and those where the symptoms.


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## relic222 (Aug 24, 2012)

brittan said:


> The leak would be in the air admittance valve in the rear of the servo unit. The servo will still work as long as the vacuum valve part of the arrangement continues to function correctly and there will be no different feel to the brakes.
> However if I'm correct you will have no reserve of servo assistance after the engine is switched off.
> 
> Some thoughts:
> ...


Thanks for this, I will do some checks and report back with my findings.


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## relic222 (Aug 24, 2012)

Okay, so the results:

1. Yes, it only starts when the engine is running.
2. It continues for a short time.
3. Yes, the hissing appears to be coming from the pedals.
4. Yep, the pedal hissed the first few times and then became much firmer.
5. I don't know what I'm doing so I'm reluctant to fiddle with my brakes!


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## brittan (May 18, 2007)

Your results strongly suggest that the fault is as per my previous post - a leak in the air admittance valve in the rear of the servo unit.

The only fix is to replace the servo and I remember a previous post on the subject and managed to find it:
viewtopic.php?f=19&t=276022

Unfortunately the person did not post the outcome but the bad news is that he was advised that on a RH drive car changing the servo is an engine out job. I cannot confirm or deny that.


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## relic222 (Aug 24, 2012)

Thanks very much for your help. Is a servo leak a big issue, what exactly will happen if it is left - as I mentioned earlier it has been like this since I bought the car (2.5 months ago) from an Audi dealer no less. The car is still under warranty, do you know whether this would be covered?


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## brittan (May 18, 2007)

If it's an Audi used warranty I would expect it to be covered under the heading: _All electrical and mechanical factory-fitted components are covered against sudden electrical or mechanical failure. _The only brake system components excluded are discs, drums & friction materials. 
http://www.audi.co.uk/used-cars/approve ... tions.html

What if you leave it? The smart a$$ answer is that it will be OK until just after the warranty expires when you'll be landed with a large bill.

The servo should not make a constant hissing noise. Get it back to the dealer and get them to fix it.


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## relic222 (Aug 24, 2012)

Will call them tomorrow! Thanks a lot for your help, I spent 2 months just ignoring it, probably a bad idea!


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## brittan (May 18, 2007)

Let us know the outcome. I'm interested in a definite answer on whether or not replacing the servo is an engine out job.


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## relic222 (Aug 24, 2012)

Booked in for Monday 26th, I'll let you know the results.


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## relic222 (Aug 24, 2012)

Dropped the car off this morning and just received a call to say that it took a while before they could get authorisation with the warranty. Fortunately that's all been sorted now and they confirmed that it's covered. I asked what was involved and he confirmed that they would have to drop the engine and gearbox as you suggested - due to the fact that it was designed as a left hand drive car. He said they've done a few before but it's a 6 hour job. Hopefully should have the car back tomorrow. Thanks for your help.


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## brittan (May 18, 2007)

Glad it's finally getting fixed.


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## ornithology (Dec 13, 2012)

Sorry bumping an old thread but was using the search. Curious if you have any knowledge if the brake servo was fautly on a LHD car (what I own) will it be an easier job to replace?


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## relic222 (Aug 24, 2012)

From what I was told it's a lot easier fix on a LHD. The guy said on LHD the part is much easier to access and the part itself isn't particularly expensive, the cost on the RHD model is in the labour of dropping the engine out.


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