Mine has done that off and on for the almost 5 years I've had the car. It sounds like a relay opening and closing, like morse code. I've often thought about learning morse code so I can decipher it but I don't have the time and so far nothing bad has happened. Its definitely not the cabin filter. The first time I heard it I pulled the cabin filter and it was clean like new. It also has nothing to do with the stock radio. Mines been removed in favor of an aftermarket and the noise is still there.
Pat