1998 3800SC . the engine is Totally stock, ALL New. starts right up, for the most part runs well. but I went to put it away yesterday and when Idling, all of a sudden it started making a loud "Sucking noise" (like a Huge vacuum leak), started to idle crappy. did this for about 10 seconds then went away ! has done this twice. Both times, by the time I could lift the hatch and look, It Went Away ! Sounds like it's coming from around the throttle body Area. (car only has 4 miles on it. ) Any Ideas ?

The IAC, when opened up by the PCM, can cause such a noise as you describe. This would be very easy to hear if you have little or no induction system installed.

But this noise you are hearing may just be the result of some other issue (causing the PCM to open up the IAC) and it may not be the cause of the problem. It would be helpful to see what the scan data is showing when it starts running bad. Specifically, I would like to see the following data:

-RPM
-IAC position counts
-o2 sensor voltage (B1S1)
-Short Term Fuel Trim %
-Long Term Fuel Trim %
-Spark Advance deg

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[This message has been edited by Darth Fiero (edited 04-20-2015).]

Was the AC on at the time? I was just thinking that will make sounds for short periods of time while idling.

Sounds like it could it could be the IAC opening up wide open then shutting, but WHY ? Have no way of getting all that info, all I have is a hand held OBD2 scanner.

quote Originally posted by Lou6t4gto: Sounds like it could it could be the IAC opening up wide open then shutting, but WHY ? Have no way of getting all that info, all I have is a hand held OBD2 scanner.

If there is a situation causing the engine to run badly, the PCM might be responding to that by opening up the IAC which will cause the noise. But the IAC opening up probably isn't causing it to run bad, something else probably is. And I would need to see some specific scan data to even begin to diagnose the issue.