Loud whistling while driving and without load on engine

[JulietTangoAlpha]

I have a '01 4.0 and when I drive down the road I hear a faint whistle but it's loud enough to hear. And when I take it out of gear and don't have a load on the engine it whistles loud and proud until I come to about 10mph. My mpgs are terrible now so I'm guessing it's a vacuum problem. I don't have any codes at all so I really don't know where to start. Any help is appreciated.

 

[CodaMan]

Does the whistle go away when you increase the RPMs? That would indicate a vacuum leak since the engines draws less vacuum as RPMs increase.

This is a VERY common problem with the 4.0. The first thing I would do is look for a vacuum leak. Visually inspect each vacuum line, if you don't see anything obvious then spray something on each line as the vehicle is running. I like to use water but others use carb cleaner. Water will kill the engine as it's sucked in through the vacuum leak, carb cleaner would have the opposite affect.

If you don't find a vacuum leak it is most likely an intake/exhaust manifold leak. Most of us 4.0 owners have dealt with this. To fix you can usually just re-torque all the intake/exhaust manifold bolts.

 

[JulietTangoAlpha]

Does the whistle go away when you increase the RPMs? That would indicate a vacuum leak since the engines draws less vacuum as RPMs increase.

This is a VERY common problem with the 4.0. The first thing I would do is look for a vacuum leak. Visually inspect each vacuum line, if you don't see anything obvious then spray something on each line as the vehicle is running. I like to use water but others use carb cleaner. Water will kill the engine as it's sucked in through the vacuum leak, carb cleaner would have the opposite affect.

If you don't find a vacuum leak it is most likely an intake/exhaust manifold leak. Most of us 4.0 owners have dealt with this. To fix you can usually just re-torque all the intake/exhaust manifold bolts.

I checked the lines and didn't see anything that suggests a vacuum leak from a line but I'll check the manifolds next. Ill post an update once I check it. Thank you


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[fourpointzero]

I had this same issue. Popped up mostly during very cold weather. When I got to changing my valve cover gasket, I torqued down the rear manifold bolts and that resolved the problem. It’s been silent for over 3 years. Hope you find your cause and resolve. It was absolutely annoying, to me.


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[JulietTangoAlpha]

I had this same issue. Popped up mostly during very cold weather. When I got to changing my valve cover gasket, I torqued down the rear manifold bolts and that resolved the problem. It’s been silent for over 3 years. Hope you find your cause and resolve. It was absolutely annoying, to me.


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Awesome thank you, and yea it's the most irritating noise.


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[CodaMan]

Not only is it an irritating/annoying noise, it's also an embarrassing noise

 

[JulietTangoAlpha]

Not only is it an irritating/annoying noise, it's also an embarrassing noise

Extremely embarrassing, I see heads turn while I drive around town.


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[Augustus]

I have this problem right now. It sounds like it's coming from the rear driver's side just below the valve cover bolts. What's strange is that when I spray some carb cleaner in the area, it stops immediately. It is driving me absolutely nuts... I would loooooovee some suggestions on this.

 

[Jerry Bransford]

I have this problem right now. It sounds like it's coming from the rear driver's side just below the valve cover bolts. What's strange is that when I spray some carb cleaner in the area, it stops immediately. It is driving me absolutely nuts... I would loooooovee some suggestions on this.

Being precise on knowing where you are spraying the carb cleaner when the whistling stops will immediately pinpoint the source of the vacuum leak. Don't just spray it around the area, give small precise shots of the cleaner at various points until it finds the vacuum leak.

 

[Nathan Stanfield]

Mine has been doing the same thing. As soon as I take my foot off the gas the whistle starts, but I just figured its a Jeep and makes strange noises.
Last weekend when I was pulling my transfer case off I realized there is a ton of oil leaking from around the passenger side of the oil pan. Maybe that will fix it.
Im going to check all the vacuum lines anyway though. Its annoying.

 

[WWII Jeeper]

Try checking the vac hose on the underside of the intake manifold, right under the throttle body. Lots of heat there, and the hoses tend to crack. My 06 4.0 had the original 90 degree rubber elbow, and the split was topside where it could not be seen.
Ray

 

[jeep_boss]

I have a '01 4.0 and when I drive down the road I hear a faint whistle but it's loud enough to hear. And when I take it out of gear and don't have a load on the engine it whistles loud and proud until I come to about 10mph. My mpgs are terrible now so I'm guessing it's a vacuum problem. I don't have any codes at all so I really don't know where to start. Any help is appreciated.


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Can you reproduce the whistle with the hood up when not driving?

 

[BigWheel88]

Curious if any of you have solved this issue for yourselves and where did you find leak. Mine started doing this and going to start investigating this weekend

 

[CodaMan]

Curious if any of you have solved this issue for yourselves and where did you find leak. Mine started doing this and going to start investigating this weekend

A lot of us have fixed this problem. Mine was a vacuum leak at the intake manifold gasket. I tightened all the exhaust/intake bolts.

 

[BigWheel88]

Turned out to be a crack in the brake booster check valve.

 

[mainegoat68]

What Jerry said. Small shots directed at a certain area, instead of a full blast.

 

[Cooz]

Was able to tighten a half turn on intake manifold bolt by firewall, Also gave a half turn on the throttle body bolts. Noise is gone

 

[dsorsini]

Turned out to be a crack in the brake booster check valve.

Not too sound too much like An uneducated newbie here, but where exactly is the brake booster check valve? I want to check this next as I have the same sound and it’s something I haven’t done yet

 

[BigWheel88]

This. The OEM one was black. Trying to find part number at Oreilly where I got replacement but not finding it.