Mystery quiet high pitch whine, VIDEO, '08 Sport

[marknreimer]

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.

https://www.youtube.com/watch?v=TrG5KrnQmgM

 

[RonD]

Cold engine
Remove fan belt
Start engine, you can run a cold engine without water pump for approx. 2 minutes, NO LONGER

Listen for the noise, without the fan noise you should be able to pinpoint the noise much better

If there is no noise then it was coming from one of the accessories, fan belt driven pulleys or tensioner, idle pulleys
Spin each one and see if you can ID the problem

Alternators can generate electrical noise like arcing when they are starting to fail

 

[marknreimer]

Thanks! Seems like a great way to narrow things down. I'll pull the belt. With a cold engine I can hear this sound right away, so it won't need to run more than 10-20 seconds to figure out if it's still there or not.

 

[marknreimer]

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!

Sent from my iPhone using Tapatalk

 

[RonD]

Good Work

Thank for updating the post