Mystery noise above certain RPMs

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driver

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Hoping that the TJ hive mind here might have some suggestions for me as I try to identify a mystery noise. I realize it's hard to offer suggestions for noises without a recording, so I'll try to nab one to update this later. I also realize it's hard to describe sounds, but here goes...

Specs: '03, 4.0, 6 cylinder, manual transmission, 68,xxx miles

Here's what I've learned about this noise so far:
  • Has been present since I bought the Jeep about 14 months ago. Unfortunately, I hadn't noticed it while test driving, making me think it's gotten louder.
  • It's a whining or sometimes screeching sound that seems to come from the front end-ish, but is hard to pin down. Definitely does not sound like a squealing belt.
  • The noise occurs when accelerating up through a gear and/or dropping down a gear and pushing the RPMs up as a result. As soon as I disengage the clutch, the noise disappears.
  • Occurs only above about 2,700 RPM when cold, but that threshold seems to drop to around 2,200 RPM when the weather is warm and as the Jeep warms up. It may also have recently been dropping overall. This is a PITA when trying to get up large hills and keep some semblance of speed.
  • Happens both under load (when in gear) and not. When in neutral and revved up to the RPM threshold, the sound is less "impactful" and more "tinny" for lack of better terms.
  • When hitting the point of the noise, the Jeep will still continue to accelerate and push up the RPMs.
  • There's no physical feeling associated with the noise as far as I can tell. No shaking, for example.
I initially thought it may have been the serpentine belt. The old one looked good, but I replaced it anyway. No change that I could ascertain.

I've read a bunch of posts from all over the place and haven't found a problem that sounds quite like this one.

I'm considering taking it to the shop to get some help in pinning this down, but I'm wondering if anyone else has experienced something similar and might have suggestions for things to look at before I start paying labor hours to someone else.
 
Sounds like the idler and/or tensioner pulleys. It's very easy to remove the belt and spin them by hand. There should be no slop or binding.
 
The change in sound depending on the clutch and the tinny sound makes me suspect an exhaust shield or the cats somehow. I might tap the exhaust with a rubber mallet and see if it makes a similar sound.
 
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pagrey said:
The change in sound depending on the clutch and the tinny sound makes me suspect an exhaust shield or the cats somehow. I might tap the exhaust with a rubber mallet and see if it makes a similar sound.
Click to expand...

I would expect those to be more of a rattling sound, right? Or could they produce some sort of whine/screech like what I'm hearing?
 
driver said:
Interesting. Sounds easy enough to check. I'll add it to the possibility list.
Click to expand...
After reading @pagrey 's response, I re-read your original post. If it was the pulleys, pushing in the clutch would not make the noise disappear. Maybe input shaft?
 
Clutch worn to the point it is on the rivets? Slippage when load forces rivets to slip on flywheel?
 
Goatman said:
Clutch worn to the point it is on the rivets? Slippage when load forces rivets to slip on flywheel?
Click to expand...
Interesting, thanks. I hadn’t considered the clutch as a potential culprit. Any tips for looking at that or testing it? Other possible symptoms?
 
There isn't a way to inspect the clutch without getting to it. You could check for clutch thickness if you can get an inspection mirror in there. Through the starter hole or clutch fork hole might do it. And if the clutch material is real thin on one of the sides you would find out for sure when replacing to worn clutch. But this is not a likley culprit. When the obvious isn't the problem then thinking outside the box can be helpful.
 
Update on this for anyone who finds this thread later when trying to identify noises.

Turns out it was the fan clutch. After tracking the sound down to the engine bay, I pulled the belt and ran the engine for a minute with it off. No noise, so that isolated it to the belt drive.

I already had a fan clutch sitting around, so I swapped it in and the noise is gone. The front end also seems much quieter when accelerating.

A bit strange since the fan seemed to be behaving properly, but I’ll take this as a win.
 
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