Loud whistle vacuum leak, can't find the source.

rockytriton

rockytriton

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Apr 10, 2024
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Location
Omaha, NE
99 Jeep TJ 4.0L

Here's a video to see what it sounds like.


When RPMs go up it goes away.

I've tried spraying every line and around the intake manifold with carb cleaner. No change in the sound when any area is sprayed. Checked all the rubber hoses around the intake manifold and no cracks, everything looks nice and solid.

All bolts on the intake manifold are tight as well.

The only thing I can think of is the two ports on top of the engine have some play, like I can move them up and down and side to side a little. But spraying them with carb cleaner makes no difference in the sounds, neither does moving them around. Is it ok for those to have some play? They are on the metal part on the top, one on the front and one on the back, the front one goes to the big hose to the airbox.
 
rockytriton said:
99 Jeep TJ 4.0L

Here's a video to see what it sounds like.


When RPMs go up it goes away.

I've tried spraying every line and around the intake manifold with carb cleaner. No change in the sound when any area is sprayed. Checked all the rubber hoses around the intake manifold and no cracks, everything looks nice and solid.

All bolts on the intake manifold are tight as well.

The only thing I can think of is the two ports on top of the engine have some play, like I can move them up and down and side to side a little. But spraying them with carb cleaner makes no difference in the sounds, neither does moving them around. Is it ok for those to have some play? They are on the metal part on the top, one on the front and one on the back, the front one goes to the big hose to the airbox.
Click to expand...

The loud whistle can indeed be indicative of a vacuum leak, but since you've already thoroughly sprayed the intake area and aren't seeing any change, you might want to explore a few other possibilities.
  1. Vacuum Hoses: Even if they look solid, it’s worth inspecting all vacuum hoses carefully for any signs of wear, especially around the bends and connections. Sometimes, a hose can look fine on the outside but have a small split or crack on the inside.
  2. Throttle Body Gasket: A faulty gasket here can also cause a whistle. If you haven’t replaced it, it might be worth checking or replacing as a potential source.
  3. Check the Breather and PCV Valve: Sometimes, a faulty Positive Crankcase Ventilation (PCV) valve or a loose breather can create a whistle as well. Make sure these components are functioning properly.
  4. Intake Manifold Gasket: Although you mentioned the bolts are tight, the gasket itself could still be compromised. It could be worth checking the intake manifold gasket for any signs of failure, particularly around the areas where you've noticed some movement.
  5. Check the Air Filter and Box: It’s worth taking a look at the air filter to ensure it’s properly seated and that there are no gaps between the filter and the airbox. An air leak in this area can also cause similar sounds.
  6. Listen for Direction: Since the whistle goes away at higher RPMs, it might be related to airflow. Trying to pinpoint where the sound is coming from by listening closely, or using a mechanic’s stethoscope (or a piece of tubing as a makeshift tool) can help isolate the area.
  7. Inspect the Ports: Regarding the two ports you mentioned having some play — while a tiny amount of movement might be fine, make sure their connections are solid and there are no issues with the gaskets or seals around them.
 
I had a whistle noise in my 2006 4.0 it was obvious taking my foot off of the throttle but I could not identify the source because it was most noticeable decelerating while off the throttle. A smoke testing did not reveal an intake leak that I could see this occurred some time after the gasket was changed by a shop in town. Another 6 to 9 months later smog testing was due again and I could not get the emissions to clear again so I started spraying carb cleaner around the manifold finding there was now a leak strong enough to show results positive for an intake leak. Taking it to a shop because I didn’t want to do it myself they found a crack in the intake manifold. I wound up buying a guaranteed no crack manifold from eBay with the same part number because no one within a couple hundred miles of Vegas had one new and none of the auto salvage yards had one. Now I’m waiting for delivery between Dec 2nd to the 6th hoping it closer to being delivered on the 2nd. Intake gasket was available at the dealer but I had to ask for next day air as it was coming from back east, next day air was only an additional $14 bringing that gasket to around $100 total and the used manifold off eBay was another $200 used.

A quick spray was not sufficient I had to really spray a good amount of carb cleaner around the manifold and then it first wanted to stall before an increase in RPM. My crack was at the firewall end around cylinder 6. The same cylinder I was also getting a p0136 with one PCM and a p0174 with the other PCM, just to give you as much info on mine as I can.
 
Last edited:
It definitely sounds like the intake manifold to me. If you can't find the problem, why not take it to a skilled mechanic?
 
I'm not saying it's definitely not a vacuum leak....but I suspect it's something else. You said when you increase the RPMs it goes away. Well rev the engine up and then snap the throttle closed. When the throttle closes, vacuum jumps up so if it's a vacuum leak it should get even worse until the vacuum drops. Also, you need to put a vacuum gauge on it and check the readings. I'd put the vacuum gauge on the hose to the brake booster.

If the vacuum gauge readings are normal, then I'd be investigating elsewhere. Personally I think it sounds like a bad bearing in something being driven by the belt. Pull the belt off and spin each pulley. See if any of the bearings have any play in them. I would check the alternator first. Might be my uncalibrated ear but from your video it appears worse at the front of the engine. Check the fan clutch.

It's going to take some diagnostic work. Start with a vacuum gauge tool and then go from there.
 
Thanks everyone for your replies. Today I again checked hoses and all were fine, checked airbox filter and was good, changed PCV valves front and back one, still no help. I suppose it could be the gasket. But also it is louder at the front, feels like it's coming from where the belt is, so maybe not a vacuum leak.

I guess I'll just call my mechanic and have him take a look. I just like to see if I can figure these things out myself first.
 
rockytriton said:
Thanks everyone for your replies. Today I again checked hoses and all were fine, checked airbox filter and was good, changed PCV valves front and back one, still no help. I suppose it could be the gasket. But also it is louder at the front, feels like it's coming from where the belt is, so maybe not a vacuum leak.

I guess I'll just call my mechanic and have him take a look. I just like to see if I can figure these things out myself first.
Click to expand...

Please post when you figure out what it is.
 
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I just had a whistle that changed a bit after warm up. I used a stethoscope to track it and even then it took a while while. I had just done Manifold and valve cover and it sounded on other side anyhow. It was a bad bearing on the alternator developing. Just something to check. Rewatched and it might be AC compressor bearing too.
 
Ok latest update fellas...

I took the advice to take off the belt and ran it briefly, zero noise with the belt off.

I put the belt back on and sound came right back, a loud and annoying as ever.

So I got the hair-brained idea to spray around the bolts of all the pulleys with some WD-40. In retrospect, that's probably not a good thing to do, but I did at least try to avoid getting any on the belt. But after starting it back up the sound mostly went away. An occasional whistle here or there that lasted a second or two. Then I drove it around the block a little bit and mostly the sound was gone except occasionally creeping back up for a couple seconds and then going away.

So anyway, this leads me to believe it's definitely one of the pulleys. I tried to spin them all with the belt off and none of them made any real noise or felt loose, so maybe it has to be spinning at a decent rate before it will start making noise. The only one that had any play was the idler pulley but that's because I loosened it to take off the belt.

I think I may just play around with it some more before I take it to a mechanic since I feel like I'm at least zeroing in on something.

EDIT: Also i noticed when driving that the sound changes when I push in or let out the clutch, but maybe that's due to rpms changing.
 
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rockytriton said:
Ok latest update fellas...

I took the advice to take off the belt and ran it briefly, zero noise with the belt off.

I put the belt back on and sound came right back, a loud and annoying as ever.

So I got the hair-brained idea to spray around the bolts of all the pulleys with some WD-40. In retrospect, that's probably not a good thing to do, but I did at least try to avoid getting any on the belt. But after starting it back up the sound mostly went away. An occasional whistle here or there that lasted a second or two. Then I drove it around the block a little bit and mostly the sound was gone except occasionally creeping back up for a couple seconds and then going away.

So anyway, this leads me to believe it's definitely one of the pulleys. I tried to spin them all with the belt off and none of them made any real noise or felt loose, so maybe it has to be spinning at a decent rate before it will start making noise. The only one that had any play was the idler pulley but that's because I loosened it to take off the belt.

I think I may just play around with it some more before I take it to a mechanic since I feel like I'm at least zeroing in on something.

EDIT: Also i noticed when driving that the sound changes when I push in or let out the clutch, but maybe that's due to rpms changing.
Click to expand...

lol I have the exact same problem on my 02' TJ right now. so annoying and pretty sure its my pulley too. I'm gonna get a mechanic to diagnose which pulley and change it myself
 
Making progress! Spin each pulley by hand. You are feeling for grittiness or "bumps" while spinning. Anything but smooth movement is a problem.

I would suspect the idler pulley first and the alternator second. If none of those, then hard to say.

When spraying a light lube when trying to find the issue, squirt a little on the pulley bearing area one area at a time until you find the pulley that changes.
 
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rockytriton said:
I think i want to just change both of those idler pulleys since it looks pretty simple. Anyone know if these are the correct 2 items?

https://www.autozone.com/external-e...ory-drive-belt-idler-pulley-231098/909940_0_0

https://www.autozone.com/external-e...a-c-drive-belt-idler-pulley-231107/634072_0_0

I can't tell if they are the same thing either as they look the same to me.
Click to expand...

Looks like you have two idler pulleys? Looking that year up on Rock Auto it appears to me that one is stationary and one is the tensioner.
 
Very nice! Glad we could help!

Take a good look at the old pulleys. Specifically the bearings in the center. You'll probably find some play in them.
 

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