I have a 1999 F150, 4.6L with 105,000 on it. As soon as it got cold here I started experiencing a loud noise. I have read other threads talking about a whine or whistle at cold temps. This is much louder. The noise will only last about 10 seconds but comes and goes at will. It may be two minutes after starting the truck when it is cold or after running it for 20 minutes. The noise almost sounds like an air horn going off under my hood. Like I said it doesn't last long. But, when it is happening the trucks RPM's will go up about 500. When the noise stops the truck almost stalls out. Went to three different mechanics. They all named different potential problems. But they all said without hearing it they can't diagnose. Like I said, the truck does it at will, just never near a mechanic. What part would be kicking on and off and putting such a load on the engine to cause the RPM's to rise. Obviously a part is going bad, just which one?? Also, I know that the AC clutch is fine. My first thought was that since I had just started using my defrost. I have heard that kick on and off, no trouble.

 

I'd wager to say that your idle air bypass valve may be sticking. Tap on it with a screwdriver handle or something next time it acts up, see if it goes away.

 

The iac is known to make a loud hooting type noise when they start going bad. Do as Quintin says and tap on the iac valve when you hear the noise. If the noise stops, replace the valve.

Robbie

 

Where exactly is the IAC? I had a noise under the hood when my alternator started going bad. After the alternator was replaced the noise stopped for a short time but now is back. Just happens on cold days; lasts 20 seconds to a minute but sounds more metallic than air-like.

 

The iac is behind the throttlebody. One of the air tubes from the air intake tube(close to the mouth of the throttlebody) runs right to it. You may want to check the idler pulleys on the serpentine belt. Ford had a message a while back on the grease in the pulleys causing a noise with cold ambient temps. They revised the pulley with a different grease.

Robbie