Recently, I heard a loud buzzing sound (similar to a motorcycle exhaust) coming from the front-end of my Expedition (2000 model, 5.4L). The sound only occurred when I took my foot off the pedal and coasted the vehicle. Then the problem went away. Concerned that there might be something wrong with the front end, I took it to the dealer, but they could not find anything.

Then the following day, I experienced the "no idle" symptom of a stuck IAC. Now I am beginning to suspect the buzzing sound might also have been related to the failing IAC when subjected to the high vacuum condition caused by letting up on the throttle.

I had to fix a stuck IAC in this vehicle 4 years ago, but I never associated it with the sound I described above. Do you think they are related?

 

Yup, replace the IAC and the problem ought to be gone. I find cleaning it might buy another year or 2, but that buzzing is a sure sign to me it's gonzo, especially if original.