Need help identifying high pitched whistling noise

Hi All.

I have started to hear a high-pitched whistling noise on my '99 XJ. 160k on the clock. Stock configuration, automatic. I am thinking a belt or bearing possibly, or maybe some sort of vacuum suck but having hard time knowing how to start narrowing it down. Since my hearing is not that good I cannot even tell which direction it is coming from but I think the front. I will have a good-hearing friend help me with that I guess.

As far as the sound itself, it almost sounds like a leak around the door seal like when you don't have the window rolled up all the way. Pretty sure that is not the cause of this noise but that is kind of what it sounds like. It is also not too dissimilar from tensioner pulley noise when they start going but not rattle-y like the ones I have heard.

Other observations: I hear it generally from 25 mph – 45 mph or so, and the vehicle does seem to need to be moving. Does not seem to change pitch or frequency with speed or rpm, and does not happen all the time although it is becoming more frequent. Happens mostly while accelerating and decel; not when coasting. I haven’t heard it at idle, even with elevated rpm. I put the rear on jackstands and with the vehicle in gear could not hear it at any ‘speed’.

A couple things I have considered:

Unit bearings. Since I am still new to Cherokee stuff, is the try-to-wiggle-the-tire-with-your-hands-at-12-and-6 test work for a unit bearing like it does for the ‘regular’ style bearing? This noise is not ‘growl-y’ like other wheel bearings I have heard.

Vacuum suck. No clue how I would start checking on this without just replacing all the hoses. I guess that might be okay with 160k on the ticker, and fairly cheap. No CEL coming on.


Belts. Again, I guess I could just replace in the name of maintenance, since I haven’t replaced them yet in the 1.5 years since I bought it. They look good after a preliminary inspection, but then again…


Any other ideas or thoughts on how to deal with this? Fuel pump?


Thanks,
Jim

Might want to tighten the belt and see, could get lucky

Throttle body gasket is pretty common source of whistling noise; the gasket on these things is thin and brittle and can crack from age/weathering, not enough to make big vacuum leak but big enough to make a loud whistling sound. The throttle body itself can whistle but that doesn't usually just happen by itself.

Crack in the exhaust manifold welds is also a common source of whistling noise.

Also the brake pads have a metal tab that makes a scraping sound as an audible wear indicator. Check those.

Unit bearings. Since I am still new to Cherokee stuff, is the try-to-wiggle-the-tire-with-your-hands-at-12-and-6 test work for a unit bearing like it does for the ‘regular’ style bearing? This noise is not ‘growl-y’ like other wheel bearings I have heard.

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Unit bearing wear can be detected by shaking it, but it won't tell you if its just dry. Look for rust dust (not brake dust) around the back of the bearing. Look for rust dust on your u-joints too, and shake them, and you can also use a small pick to lift the seal on any that look good and look for dust there too.

Vacuum suck. No clue how I would start checking on this without just replacing all the hoses. I guess that might be okay with 160k on the ticker, and fairly cheap. No CEL coming on.

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Vacuum leak makes the engine run poor--uneven idle, chugging, that kind of thing. You can get a simple test by pulling something like cruise control vacuum hose and use your finger to test for suction. Or just buy a vacuum gauge at VatoZone or any of the others, less than $20, healthy warm should put 15-20 hg constant vacuum pressure.

A foot-long section of 1/2" coolant hose, stick one end in your good ear and use it to help isolate sound.

Very helpful answer. Thanks for the insight. I will start checking those things out.

Last time had it in the shop the mechanic wanted to clean the throttle body anyway so I may go ahead and get that done and get a new gasket in there along with that cleaning.

If it's not really tied to road speed, I would suspect something in the engine bay.

Out of curiousity I'm kinda thinking you should check your mech fan, pretty easy turn car off, open hood, move fan back and forth with fingers. it should move in a mushy fashion with a little bit of resistance. When these go bad, usually the car sounds like it's hauling ass, zaps power, and can cause belts to squeal.

While you're at it, check the idler pulley and bearing. Again, car off, grab it and wiggle.

If you have screwed with the air box and/or hose, you could be getting a whistle. It was quite common back in the day when people pulled out the restricter.

I thought I would reply to my own post, with the resolution.

Turns out it was the ski racks I had recently attached. So my original post I said it sounded like a door seal, and in the end this was probably the closest thing after all -
an aerodyanmic issue. The whistling noise was just air moving around and through the narrow gaps.

Now that is the kind of problem I like best - nothing broken