Hello, each time my iMac shuts down, I hear this noise coming from the speakers or fan. I hear it louder when plugged into a speaker set. The person on the Apple Care Support call said that there might be an issue with the fan, but I think it could be something else. Any ideas what it is? I have attached a link to a recorded video: https://photos.app.goo.gl/Rj5gfy1qK4KxtTpU2 Please help me out.