E320 CDi annoying sound on cold engine!

[skemball]

2000/X 75k miles e320 CDI - the engine has developed a prominent deep resonating sound between idle and 1300RPM when cold. As the engine warms up it gradually becomes quieter.

It not coming from inside the engine (pistons, bearings or valve gear etc.) much more likely from somewhere within the air intake systems - it hasnt always done it and the engine has full MB service history - last service a couple of months ago - .............. hhmmm interestingly im sure it wasnt like it before.

any ideas anyone??

 

[blassberg]

It not coming from inside the engine (pistons, bearings or valve gear etc.)

Without that info I would have said glowplugs wearing or oil level ok but down a bit.

Glowplugs would be accommodated by a darker / smellier than usual exhaust.

I do wonder if the slightly rougher sounding engine caused by one of these options is creating a resonance or a non-resonance issue in something that can transport the sound better eg airbox / pipes acting as a soundbox

cheers

 

[Brian WH]

Resonance sound - Could be a loose airbox which when hot tightens up?

 

[996jimbo]

Possibly an exhaust beginning to blow or exhaust manifold - closing up (at the moment) as it warms?

 

[skemball]

Thanks for responses - I have checked the air box - not coming from there - checked the exhaust for tightness etc. not coming from there.
I think it may be some sort of auxilliary air valve in the intake air stream that operates when the engine is cold and is resonating. Its one of those noises that not 'mechanical' and cant be pinned down as to anywhere specific. I have tried to locate the source using the stethoscope technique with tubing and wooden sticks etc.