The purge valve is a very noisy unit on our newer cars.
On the 01+ C-classes, there was an updated purge valve
line bracket that secured the evap line with rubber instead of solid plastic to help isolate this noise from the body.
MB is aware of this similar issue on the new E classes and
I belive they are working on a revised bracket and evap line
clips to isolate the valve from the body. ( Or at least that's what the Tech Specialists for MB have said.)
I don't know when these will be available. Soon I hope.
I would continue to have your dealer put heat on their Factory MB
representitive. This can only help speed things up.