Just bought my first merc yesterday - E240 Avantguarde 04reg. Great car to drive but I noticed a pump/compressor noise when I break/slow down after cruising around 50MPH for 5mins or so. I can feel the pump vibrations through the break pedal. What is the noise? Why does the compressor turn ON?

 

it is the sbc brake system,known defects include pump noise.The dealer may have a service recall or retro fit solution.It is a common problem with the fly by wire brake system.Get it checked.
ohlord

 

I get so used to this noise in the W211sIeveb likeit now. Just check if the sbc unit in your car has been already replaced by a dealer.