Vito 108 Cdi diesel "pinking"

[orange vito]

Yes i know diesel's dont pink but my Vito seems to be doing just that:-x
It happens when pulling away from rest in first or second but disapears when revs rise above 2000.
Not actually sure it is pinking but its the best way i can describe the sound.
So far changed the air,fuel filters and oil and filter. Used injector cleaner and used BP ultimate fuel all to no avail.
Just to make it harder to fix it doesn't happen all the time and is usually accompanied by rough/lumpy tickover but no excessive smoke from exhaust
FSH ex RAC van with 64,000 on clock.
Help!

 

[hmang]

just nicked this from ba9rns link in parrots how to clean egr post


If EGR is on during idle, stumble and even stalling will likely result. If EGR is on during hard acceleration, low power (from reduced air/fuel volume) is the result. At part load, lean mixtures for economy create high combustion chamber temperatures and without EGR, these conditions not only create Nox, they also foster pre-ignition and pinking.