I did the 2.5 to 2.2 swap a couple months ago and initially had some problems with it dying at cold startup or idling at 2 - 2.5 k and then starting to miss like crazy and throw CEL (which I guess if the computer detects high idle for a minute, it will shut down a cylinder). I used seafoam to clean the IAC, which appeared to resolve the problem. About a month after that, the CEL popped on again with the same codes (no problems idling or dying though). I cleared them and they have not returned over the past month. However, I have noticed once or twice that the IAC valve is humming or buzzing after the car is shut off. The first time I noticed, I just heard this buzzing after I got out of the car and looked around the engine for a bit... then when I disconnected the IAC electronic connector, the sound stopped. I had a spare IAC from the 2.5 Liter engine I had removed and I plugged it in, and it buzzed as well in my hand as I was holding it... looked like the valve was vibrating really quickly back and forth a very minute distance. I left it for awhile and came back and it stopped. I also havent noticed it again... I've only noticed it twice in 3 months and doesnt appear to be any issues with the engine running (although I get 23-24 mpg in city or on highway). Just figured I'ld post to see if anyone thought I should worry about it.

 

Thanks!

That's normal. It vibrates untill the PCM shuts down, then stops.

 

I'd look for air leaks between the MAF and the throttle body. Any of the PCV hoses or the IAC hose can leak and let unmetered air in. The engine will run fine when it's throttled up, but it will idle funny.