Drivers of 2006-07 Ford Freestars and Mercury Montereys may come into your store complaining of a popping sound from the engine compartment at idle.
Once you have the hood open, you'll probably notice that the noise is coming from the air cleaner housing. The cause of the noise, says Ford, is the idle air control valve opening and closing a bit too fast, coupled with the design characteristics of the air cleaner assembly. The two together result in abnormal resonance setting up in the air cleaner, and that's what leads to the popping noise.
Reprogramming the PCM with updated software is the only surefire remedy. Check with a dealer for the specifics of the flash.