Strange noise....

[Blocco A Spiral]

Hello all, I've recently bought a used A6 avant 1.9tdi (AJM engine) which is going just fine apart from a strange intermittent noise and I'm wondering if anyone can help identify its origin.

The noise only seems to occur at idle and is a regular low frequency thump accompanied by a short rasping sound, it does this about 3 times a second and is not directly related to engine speed. It goes away as the revs increase but does not necessarily increase in frequency. The rasping noise seems to originate from behind the oil filler near the top of the engine.

I got the impression that it was caused by a release of pressure somewhere and figured that the only place that has any kind of air pressure at idle is the brake servo circuit. When I pump the brake pedal the noise goes away.

I haven't had time to investigate yet but was hoping the someone may have an idea what might be causing this. The car doesn't drive any differently.

Also, and this might be normal, when the engine is switched off there is a hissing noise from the brake servo/ back of engine area that continues for a short while. Any suggestions ?

[Peter D]

This could be the Vacuum pump for the servo and or the one way valve that retains the vacuum. If you have and auto box then there is an electric vacuum pump thet turn on under low vacuum and the this may be pulsing due to a faulty vacuum switch. If you leave the car turned off for 2 mins is the brake pedal still functional or has the vacuum leaked away. Regards Peter

[Blocco A Spiral]

Thanks for the reply Peter. I've located the source off the metallic rasping noise, it's the "Mechanical Exhaust Gas Recirculation Valve" that also seems to be causing the low frequency pulsing sound (I assume in the inlet manifold and pipework).

I don't know whether the EGR valve is the culprit or whether a problem somewhere else is causing the valve to operate in this way.

[Blocco A Spiral]

Aha, did a search on EGR and found this: https://www.vwaudiforum.co.uk/forum/s...ead.php?t=1581
Sounds like what I'm hearing but without the rasp so maybe cleaning the EGR will cure the problem