Single knock.

Alright so its been like 2-3 weeks since I been hearing this kinda loud single knock everytime I stop the car. Like for example, when I go to school, I turn off the car when I get there and get out, and within 3-5 secs of turning the car off i hear this single knock in the front part of the car as if something was turning and was suddenly stopped by something but theres no more noise. I thought it had something to do with engine rotation but today something different happened.

Today I got in my car, I open the circuit without turning on the engine and started listening to some music. I was about to turn my car on but someone called me so I put the circuit off and get out of the car and I hear the same noise which means it doesnt have anything to do with the engine, right? It must be something else. What else could it be? Thanks.

What year car? What tranny? What engine?. You will get a much better response if you include as much info about the vehicle as possible, Off the top of my head it sounds like either a fan or a rocker.

2001 Pontiac Firebird V6 3.8L

Also, I *never* use my AC. So you cant circle that one.

Headlight motor maybe.... maybe a evil gnome is trying to get out from under the hood..

How far up front does it seem to be coming from? Could the noise be coming from within the dashboard?