Heater Problem???
 

When I turn on the heater of my 300e, I can hear a gurguling sound out of the central vents....I sit there and wait for water to pour out, but nothing comes out. I don't see any visiable leaks, my coolent levels have not lowered, and the heat from the heater does not feel damp, nor does my windows get fogged up like they would if a heater core went. Does the mercedes system, only pump in the water when the heater system is turned on, and is that why I hear the noise? or is it a constant system like older USA cars?
thanks