Volkswagen Transporter making a knocking sound - what's the cause?

My 2011 Volkswagen T5 Transpoter Kombi has started to vibrate. It has also made a few knocking sounds. Both disappeared when coasting in neutral or keeping a constant speed by putting my foot gently on the accelerator.

This has happened twice over the past few weeks, but I can't seem to reproduce the problem to show my mechanic who has checked over the basics - wheel bearings, suspension etc. Nothing coming up on the computer scan either. Can you offer any explanation?

Asked on 4 February 2016 by Rod A

Answered by Alan Ross
We would suggest that you check out the vacuum pump as if faulty is known to cause this type of noise. It can be the push rod not following the camshaft.

Start the engine and if the noise is there ,then pump the brake pedal ,if the noise goes then it is the Pump.
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