We have a 2007 Escape 4cyl with about 120k on it. It runs great, and the AC blows cold. However, whenever you turn on the AC, and come to a stop with the car in drive, there is an obnoxious banging noise( like someone tapping metal) emitting from the engine. You do not get this when the car is in park. Nor when the AC is turned off. I stare at the engine while it is running, but cannot decipher where the noise is coming from. Any insight is appreciated.