If you have your heater set to on when you turn on the ignition (but not the engine), there's a small electric pump that might be running. Either under the heater valve or near the bottom corner of the rad (on e39s, not sure where it is on the e46 but assuming somewhere similar).
Also, the air conditioning system has blower fans as well as a small fan to sample cabin air. These might be running as well.
Last edited by pleiades; 12-12-2012 at 12:04 PM.