P38 Diesel engine noise @2500rpm+

Hi All,
My P38 2001 DSE 128k miles - has started making an annoying engine noise (like a light /muted knocking) at approx 2500+ rpm. It wasn't really a problem until I got on a motorway and once going over 70 mph @ 2500rpm there was a nasty vibration - like loud road noise. At the same speed, but lower revs noise goes. It was one for the benefits of my RR driving over 68mph - unlike my SIII. Any thoughts?
Thanks

Does it do it if you hold the revs at 2500 with it in neutral?

Thanks Wazzajnr - I'm pretty sure the engine noise is there in neutral, but obviously not the vibration/road resonance - I will double check in the morning and come back.

Wazzajnr said:

Does it do it if you hold the revs at 2500 with it in neutral?

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Without listening to it very hard to diagnose. But have a look at the crank damper make sure it's secure and not breaking away from it's centre.

It is related to engine speed and not road speed?

Thanks All,
It does make the noise in neutral (though what happens at 70+mph is a much worse vibration/resonance). It's not bad YET, but is worth checking out, so I'm taking her to my local Landy Whisperer next week for some mind-melding. I will mention the "crank damper" and also been told it could potentially be; fuel pump starvation, a cracked outlet manifold plate?, even the new Avon Z1 tyres fitted! - So I will let you know what transpires (I understand the anticaiption will be draining...). Thanks again.