I've been posting about a strange high pitched whistling sound occuring when I take my foot off the gas above 2,000 rpm in my 2000 F-150 XLT with 5.4L V8. I spent the entire day today under the hood checking every vacuum line and engine accesory for possible causes of this noise. I discovered that my alternator bearing was squealing so I replaced it and was hoping that if not the bearing than maybe diodes were the possible cause of the noise. After installing the new alternator, the noise was still there. Following the advice of a tech at my local Ford dealership, I took the IAC valve off and cleaned it. I put it back on and noticed that the noise had been reduced. I then got a new IAC at Autozone and after I replaced it, wouldn't you know- problem solved.

Just thought I'd make a post so anyone who has this problem in the future will hopefully be saved of the weeks of aggravation that I've gone through. It's important to note that the IAC valve appeared to be working fine. The truck idled great and I even tested the IAC with an ohmeter and everything was ok. But for some reason, it made an awfull whistle noise at just the right amount of vacuum, and the only solution was to replace it. Also, based on input from a couple guys from this website, the symptoms were very similar to the regulator in the heater hose. The main distinguishable difference from what I can tell is that my truck did not make this noise when revving the engine in nuetral- only when in gear and under load. The guys with the heater hose noise reported that the noise was constant at a certain rpm, even when revving in neutral.

KEYWORDS: high pitch pitched whistle whistling noise sound screach squeal decelerate