I have C class C160 coupe with a strange purring noise

When i sit in the car and put the key in the ignition there is a purring noise for about 30 seconds then stops. When I stop the car and turn off the engine again this purring noise starts for about 30 seconds and then stops. It is coming from the heater control area on the dash.

Any ideas what this is??

 

Cars with SAT/NAV systems often run a fan for a bit when first starting or shutting down. I think it's done to get the temp of the computer components into a working range (warm it up a bit when starting, and cool it down a bit when shutting down). My wifes Range Rover would "sense" an approaching key and would do the same thing, bringing the comp up into standby mode. Down side was if you left a key in range of the parked car, like on a hook in the garage, the darn system would discharge the battery overnight.

 

If might be a vacuum pump. My '94 C280 had a vacuum leak under the dash that caused the pump (in the trunk) to "purr" more than it should. I'm not familar with your model though.