I am a member of the Mercedes Benz Club of America, Lone Star Branch, and we had a tech session at our local Benz dealer sometime back where this was discussed. The dealer tech rep said that "lots of stuff continues to go on under the hood when the car is shut off, and this is normal". For example, fans continue to run to purge the cabin filter system. I would suggest you let the dealer's rep listen to the noise to see if it is "normal" or not. This assumes the car drives without other problems, and this is your only concern.