I would be very interested in hearing what your dealer has to say about the noise you are experiencing, as my wife's E350 wagon makes the same noise. I notice it more on cold starts and occasionally when starting the car after it has been driven and then shut down for more than 30 minutes.

The service advisor at the MB place here said not to worry about it, but it is not a pleasant sound, even though it doesn't last long.

There is a post or two on BenzWorld about this, but no one has reported back with a resolution to the issue, except one reply which states that replacing the "air pump" made the sound go away????

Good luck.

__________________
Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013