Mercedes S350 W222 whining noise when braking

[Graham-ur8ib]

Hi there...I'm getting an intermittent whining noise when I press the brake pedal...the noise seems to come from behind the dash/center console...its not from the drive-train..a quiet whine.....goes on and off with brake pedal when pressed softly eg parking speeds........Merc service department have heard my recording of the noise (on phone) but say they don't know the cause and without experiencing it first hand cant do anything g about it...very frustrating as its an intermittent problem.

Ideas anyone ?