What can possibly be causing the humming sound from the engine on my CLK500. When I start the car up after it has been sitting out all night in the cold, the engine hums when it is idling. After I put it in gear and drive away the humming goes away. What's up with that. I am not referring to the ticking sounds that some people are complaining about on the new CLK500's and new E500's

 

That's the electric air pump running.

 

Thank you for your response.