You are correct, the pump will run to provide pressure to "draw" in , it will then reverse to reset the elements, you will hear the sound in the trunk. The pump will also run to pressurize the lumbars/ reservoir, and lastly it will again run backwards to build up vacuum assist for the HVAC/reservoir.
I think it also runs randomly to remove any moisture that has accumulated, its something I read recently!
And I found it, its on the 140 MB DVD
One minute after turning on the ignition, the pump runs for 2.5 seconds and supplies vacuum to all the elements, then twice at 5 minute icrements, and then once every ten minutes. After closing all the doors and things it, runs again for 2.5 sec providing vacuum to all the elements. Only MB would have engineered it this way!