Weird hum/whistle.

[JA no Y]

I have a weird noise coming from the 4.0L engine on my 98 Explorer. It sounds like a mix between a low pitched whistle and a high piched hum. It doesn't do it all the time, usually while idling, decelerating, or low speed city traffic. The sound goes away when accelerating, or driving at highway speeds.

At first I thought it was coming from my air box or somewhere around my MAF sensor. But once I removed it, and the intake air hose, the noise was coming from the intake. I used carb/intake cleaner and sprayed it around the intake gasket while the engine was running to see if there was a leak and maybe it was sucking in air. Listened for any change in the idle but there was none. Tried it when the noise was present and when it was quiet.

I don't want to start taking it apart looking for the source of the noise until I have an idea what to look for. I use this truck daily for work. If it's not running, I'm not working. So any advice would be MUCH appreciated.

Also, not sure if this is related to the humming, but I've noticed that when I accelerate up a hill, it makes a ...well...best way I can describe it is, it sounds like someone is throwing poprocks into a can. The engine doesn't studder or loose power, no change at all, just sounds like small backfires.

[rangerreb]

Sounds like a bad IAC valve (Idle Air Control Valve). I have had this problem on several fords in the past with the 3.0 and 4.0 ltr engines. I have removed the valve and cleaned it with carburetor cleaner but, I haven't had much luck with cleaning; I usually have to replace the valve. RB

[shorod]

The IAC does sound like a likely culprit for the whistle.

As for the poprocks in a can, that might be due to a broken weld on a catalytic converter heat shield.

-Rod