I believe this issue is actually the boost solenoid failing, it still boosts but removing the vac line to the wastegate actuator stops the noise (but would only run the low spring boost.

As the EGR uses the identical solenoid I am going to swap them over, I believe the solenoid also pulses to maintain a certain pressure on the turbo, its been doing that for the last 235k miles so my thought is that its slightly tired now!

I will be installing a manual boost controller at some point but would be looking to use that as a VNT/VGT controller when I go for a turbo upgrade in a year or so time.

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UK spec Mercedes W210 E300 Turbodiesel wagon - OM606.962 with 722.6 transmission - rust free!