Whirring noise when I switch of the engine- any ideas?

Could it indeed be the fan(s)? I believe that the engine fan runs most of the time, and the aircon fan runs when the aircon is in use. When I switch off my A3, there is the noise of either one fan or both fans spooling down. To check on this, open the bonnet before switching off, and get someone to switch it off while you are looking into the engine compartment. I guess if the engine is not hot enough, then no fans will be running, and thus no noise. So depending on engine temperature and aircon on/off, there will be a difference in the noise when switching off. At least that is what I have observed