With the car on but stopped and the fan on (AC can be on or off) I hear a very quiet knocking/tapping noise which isn't precisely rhythmic but seems to be every 5 seconds or so. It kinda sounds like someone is tapping the bottom of a steel cooking pot lightly with rubber hammer. I can also feel this "tap" slightly through the brake pedal (again, sitting in car stopped at idle with foot on brake). This goes away once this car is fully warmed up. It also stops immediately if I turn the fan off regardless of it the car is warmed up or not. Any thoughts on what this may be?
Thanks