Hissing sound when throttle applied

[Noddy 99]

Hello, chaps.

2009 V6 E280 OM642 diesel:

When I apply throttle pressure, you can hear a slight hissing sound sitting on top of the engine note. It's only just started doing it and I've done nothing to the car which would produce such a symptom.

Here are the conditions:

* It is NOT injector blow-by and there is no "black death"

* It is not dependent on either road speed or engine speed; just throttle position

* It is only apparent on gentle throttle pressure - accelerate further and the noise goes away (perhaps still there but inaudible due to the louder engine note); if you back off the throttle, the noise goes away

* The car is otherwise running fine - there are no performance issues and no warning lights

I looked under the bonnet while my son revved the engine but couldn't hear anything apart from the cold-start diesel clatter. If it's related to a warm engine, I'll have to test it again later.

Any thoughts?

Thanks in advance.

 

[Cole@MBS]

Check for oil on the drivers side boost pipe, the big black pipe, that’s the most common for splitting and letting boost out!

 

[Noddy 99]

Check for oil on the drivers side boost pipe, the big black pipe, that’s the most common for splitting and letting boost out!

Much appreciated. Could you possibly show me a picture or describe exactly where the pipe is? There are lots of big black pipes and, knowing me, I'll spend half an hour staring at the wrong one...

 

[mioba]

Boost pipes have bigger diameter and have a metal collar on them, not just your normal worm band type fastener, but a more substantial clip fitting mechanism.

They tend to go where the rubber meets the collar,

 

[Noddy 99]

Sorry, I'm no mechanic. Whereabouts in the engine are we talking about?

 

[Frontstep]

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[LostKiwi]

Looks like this:

That's the passenger side one from my R Class.

They are the biggest fattest pipes going from the engine towards the radiator area and there is one each side.