Hello everyone!
For about a month my jeep has been making a very annoying whistle. It does not do it all the time and is pretty random. It will mostly do it on a cold start after about 2-5 minutes of driving, but it also does it sometimes after driving to work which takes about 40 minutes. It will only do the whistle when my foot is off the throttle. As soon as the throttle is depressed it stops immediately. It will do it all the way down to idle. I cannot seem to locate the exact source, but it is for sure coming from the drivers side of the motor. I think I have narrowed it down to around the throttle body region. Also I have noticed that it seems to be triggered by some type of relay. I say this because when I'm trying to locate it I here a click and it will stop making it. It's not the electric fan because its not on when it makes the noise. It will do it with the ac on or off. I took some short videos which I will link. I want to say this is vacuum related, but I do not see any cracked hoses or missing hoses anywhere. Thanks in advanced! I love this forum. I used to be on here all the time in high school.
JEEP INFO:
1996 XJ
4.0
AW4
VIDEO LINKS:
https://youtu.be/7a8XwtVwIAo
https://youtu.be/LxAlJb__GJs

Hey Dylan!
You might be able to isolate the problem by spraying small areas around the intake while its running with water from a spray bottle. (like this section of hose, then that section, then the top 1/2 of the intake boot, then the bottom, etc) and listen for a change in the whistle or change in idle. Could be a cracked intake boot, rotted vacuum line, loose connection, could even be a pulley bearing. Might help you find the area of the problem. I use throttle body cleaner or brake parts cleaner, but do it on a cool engine and BE CAREFUL cuz the exhaust manifold etc are right there and that stuff is wicked flammable. try water first. Good luck!

Sometimes the intake manifold bolts get loose and cause it to whistle, you can try tightening all of your intake and exhaust manifold bolts.

Do this^^ just cuz they will be loosened up anyway.

Hey! Thanks for the help. I tried looking for exhaust leaks and came up empty handed. Turned out a couple of the manifold bolts were loose. One wasnt even finger tight. Tightened them up and the noise has not come back!

Everyone with a 4.0 needs to check those bolts if they never have.

My XJ makes a noise similar to this. It changes in volume, pitch, its very weird. Only mine does it randomly no matter the circumstances. RPM, speed, TP, radio, nothing effects it.It just comes and goes with no real cause that I can find.

Have you checked the bolts?
Sprayed with carb cleaner?

If it was the intake, wouldn't it be rev dependant?

Check the bolts!!!

Okay okay haha. I'll check them when it isn't -10 degrees outside since I don't have the luxury of a shop or garage like most of you guys. Im planning on putting a header on my XJ this summer to replace my cracked stock manifold so I'll probably do it all at the same time.