noise after shutting down???

Has anyone else noticed that there seems to be a noise, like a hamster wheel, under the hood just after you get out of your car. By the time I walk past the front of the car and into my house, it pretty well has stopped. I hadn't noticed it until recently. I'm figuring it has something to do with the heater/defrost?
maybe a fanbelt that needs lubricating?
any ideas?