humming sound on 1999 Infiniti G20

after car is turned off, there is a humming sound that contiues for 15 secs.

4 answers
Is this humming sound from the engine compartment? If so it is probably the electric cooling fan on the radiator and this would be normal. If inside the car again I wonder if it could be related to the heating and air conditioning fan that is switching off but you hear it rotate while is switching off.
There is a humming sound that continues constantly after it is shut off
Fuel cap - pressure equalization. Normal.
If sound is from rear of car it could be the antenna retracting or attempting to retract