I have just recently noticed that after I shut down the aircraft to cold and dark with absolutely NOTHING running and all power (ground, APU, battery) to off. I still hear a fan or something that sounds like air blowing. All my air systems and fans are off and of course there is no power to anything in the aircraft. There is no sound on the exterior but only on the interior. Is this as it is supposed to be? What then is this sound? Cold and dark should also mean QUIET unless ground services or something is connected. Any feedback on this will be appreciated.

press 'q' two times ...it should disappear ... its a bug within fsx and not the ngx

that's an FSX problem !! Press the Q key twice to reload the proper sound files.

Thanks. I shall give that a try.