4.0 SOHC chirp, not cam syncro, so what then?

[marknreimer]

I've got an '08 4.0 6 cylinder SOHC Ranger, Sport 4x4 model, automatic.

Has about 240,000KM. Runs really nicely, but I have a little bird-esque chirp that only shows up occasionally, and only while idling in gear. It kinda sounds like a cheap computer fan squeaking, fast high pitch chirping. Immediately goes away when I apply the gas. Never hear it while driving. Never hear it while idling in Park, only while sitting at a red light. It goes away if I shift from Drive to Park, then comes back when I go back to Drive.

Seems to me like it's originating toward the passenger side of the engine bay. I first thought it was an idler pulley bearing, but it sounds to me like it's coming from the back of the engine bay. My first thought was the cam synchronizer issue so many rangers have, but then realized that isn't a SOHC issue as my engine doesn't have one!

Are there other common possibilities, or things to look for?

 

[adsm08]

The failure you initially assumed is more common on a 3.0, but an analogous part does exist on your engine. I call it an oil pump drive slug, but I don't know the proper name.

It sits where the syncro would be on an OHV, and meshes to a gear on the jack shaft, which is in place of the OHV cam, and runs the oil pump.