I have had an intermittent problem for some time that has recently become quite regular. I will hear a moan (either while driving or idling) that starts quiet and gets louder. If I let it run its course, the engine will smoothly/calmly stall. Then I can start it up again right away with no hesitation and it runs fine.
If I give it some throttle rather than letting it run its course, it will not stall and the sound stops with the engine still running well.
I have cleaned the MAF sensor and air filter (K&N). But that did not help.
I have not found this exact behavior discussed anywhere online. Any idea what might be causing it?
RESOLUTION: I ordered a new IAC (the one meant for the Bullitt, not the GT) from Amazon on Friday with guaranteed delivery on Saturday. It arrived before noon on Saturday and I swapped it out. I also disconnected the battery while doing this as some have suggested the computer needs to reset its calibration of the IAC when it is cleaned or changed. So all bases were covered. It has been running perfectly since.
There are definite differences between the GT and the Bullitt IAC valves. So, while the GT will work on a Bullitt, I suggest making sure to get the correct item for your Bullitt (Motorcraft CX1808 Idle Air Control Motor) when replacing an IAC unit.