Hi guys,

I have a 2008 Sport 4.0L V6, SOHC. 230,000 KM on it.

Today I noticed a high pitched sound coming from under the hood. It seemed to disappear when I let off the gas. Didn't get louder or change pitch with gas, it just appeared and pretty much stayed constant. It is there at idle as well, so I was able to get a video when I got home.

I don't think it's a belt or pulley, but maybe I'm wrong. I tried giving the belt a squirt of WD to see if it changed, but nothing happened. Belt is tracking straight on all the pulleys so far as I can see. The sound seems to be coming from further back, almost like it's right on top of the engine.

I used a mechanics stethoscope but couldn't find anything that gave it away. I climbed right up on top and put my ears all over the engine bay, but it just seemed to be everywhere.

If I 'blip' the gas, it stopped and returns when the RPM returns to idle.

Any ideas or things to try to eliminate?

Have a listen, it's most clear around 10 seconds or so. Gotta have the volume up.

Air flow. Whens the last time you cleaned the throttle body or IAC valve? Sometimes when things get dirty and restrictive they can cause whistling. You could also have a slight vacuum leak. That's what I would look at first anyway.

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1995 Ranger XLT 3.0L OBDII
Trans: A4LD (E) 2WD
Rear: 7.5 / 3.45 NL LHD
Tires:GY P235 75/R15 (orig 225 70/R14)
Long Bed, 20 Gal Tank
Up to 229,900 miles

I've never done that actually... BUT it's only because I bought the truck two months ago. No way of knowing if the previous owner did either. I'll look into that, thank you for your suggestion, I appreciate it!

Question.. if you rev it up a little does it change in speed or does it change in pitch at all? I think it could be a failing bearing in one of the pullies or tensioner, possible even the ALT because my ALT started to make that same exact noise before it got worse and without any loss of charging conditions or abilities. I could be wrong and I wouldnt ignore or discount what @Grumpa said either

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The pitch never changes. It doesn't rise and fall with engine speed or anything like that. If I rev it up a little while in neutral, the sound will momentarily disappear entirely, but quickly re-appear. When I'm driving, it's pretty much there all the time if the gas is being pressed or cruise is on, but if I'm coasting I can't hear it.

I also suspected pulley bearing but the fact that the sound doesn't change at all with varying speed makes me suspect..

I removed the IAC valve yesterday for a quick peak and it was definitely very dirty, so I'll clean that out tomorrow when I'm at my shop. I'll also clean the throttle body and spray some carb cleaner around various intake areas to see if I have an air leak somewhere. And of course remove the belt and run it for a few seconds to see if it is actually a pulley/bearing. Thanks for the suggestions

Think it's fixed! I removed the IAC and it was very black. It seemed to open and close ok, but I flushed it with throttle body cleaner and used q tips to clean it out.



Then I started the truck and sprayed into the throttle body on and off for maybe a minute. The throttle body did not seem overly dirty and oily inside to me.




Put it all back together and had a listen. No sign of the high pitch whine anymore, and the idle dropped down to 750, nice and smooth. I'll keep and ear open next few days but this seems to have been the fix. Thanks guys!







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Good to hear.. great advice @Grumpa

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My Build Pg. Threads
+ 93' Ranger Overlanding Build
+ 94' Ranger Bed Overlanding Trailer Build
+ 12° North Industries ~ I Support