pump priming noise?

[Bjm34new]

Last night while washing the old girl (C350 W204) I had the bonnet up doing the strut tops when I noticed what sounded like a pump priming itself about every 10 seconds, this time slowly got longer until it stopped after about 35 minutes.
The engine was warm 30 minutes since shut down and the keys were in the garage so no ignition on, I checked when I got to work this morning ands its there again now it could of done this since day one but have never noticed it before
just wanted some piece of mind really

 

[JimboDS]

Mine does the same. I did post on here asking if anyone knew what the car was doing (not worried but curious) but still none the wiser.

Hope that gives some peace of mind. And if anyone knows what the car is doing I'm still curious!