I got home tonight and after I shut down, there is a weird grinding/whiring sound from under the hood. Popped the hood, the sound seems to come from under the coolant tank. Stopped grinding and stayed whiring. Went inside, ate dinner, came back out and it's quiet.

Any ideas?

 

May be the secondary air injection pump. Relays stick sometimes and pumps shorts sometimes.

 

Wouldn't that throw a code?

 

Probably won't throw a code until you drive the car on a cold start up. And will only throw a code it the pump failed. If it was just a stuck relay it may not throw a code. Been a dealer tech for 12 years at Mercedes. Just a guess. That's the only thing in that area that I can think of.

 

The only other thing I can think of is possibly the aux coolant pump. These are just guesses for you to try to locate the noise. Very hard without seeing the car.

 

Airmatic pump down there too and those relays can stick as well.

 

+55
I don't know about the 63. When I turn off the engine and turn on the REST/AC function the Auxiliary water pump starts buzzing/faint grinding. It Is very close to the radiator, almost beneath it.
Also a W163 owner told me that sometimes the pump works automatically after turning off the engine to recirculate hot coolant out of the engine block and prevent hot spots while the engine is off