Hey I have a 2000 XJ and it whistles really loud in the engine bay but I can't figure out where it's coming from. My vacuum lines look good so I don't know where else to look. It's been doing this for a while but only happened in the winter and went away once everything got warmed up, unless I was going down the highway on a really cold day then it'd come back. Now it does it all the time shortly after starting it and tends to go away if I drive for a while, one thing I noticed is it usually goes away faster if I crank the heater on max to the floor. Any help on where I should look is much appreciated

Edit: It sounds like it's by the firewall

 

Sometimes the belt can make whistling noises when something goes wrong. Inside the firewall, there is the blower motor (it blows hot or cold air). When they are going bad and turned on, they can make a whistling or screeching noise.

 

Gotcha. I know my belt needs replaced but it sounds like it's coming from another area. Also happens when the blower motor is on or off, really weird. I honestly didn't mind it until recently because its gotten a lot worse in the last few weeks.

 

Weird. After reading your symptoms, I thought it was for sure the blower motor since something similar happened to me. Is the noise coming from the passenger side or the driver's side? Does it have the noise when the car is stopped? Maybe there is a hole in the exhaust manifold that gets smaller when temperatures change, but that normally doesn't make a whistling sound.

 

I have a whistling noise because I threw my throttle body on a lathe and straight bored it as far as I could. It now whistles because I didn't round off the IAC port. It sounds too cool to fix at this point.

Yours might be because of a broken vacuum hose. They generally snap by the passenger side at the firewall, bit it makes the AC only work on defrost.

 

A bad vacuum manifold gasket will give that loud whistling noise.

 

Happens when I'm stopped or coasting mostly. Tends to go away when I give it throttle. Also it'll go away and then if I'm driving down the highway and the air is cool it'll come back again. It sounds like it's in the middle or slightly driver side.

 

Bad vacuum manifold or exhaust manifold gasket?

 

It usually comes from the seals on the throttle body butterfly shaft... only happens at high vacuum... fast coasting in gear with foot off the gas. If you can make it happen at idle. Spray both sides with soapy water..watch it go away

 

Alternate theory: brake booster vacuum leak. You could test it by removing the vacuum line at the booster and blocking it off to see if the whistle stops. You don't really want to drive it like that unless you know what you are doing and can stick to 25 mph side streets with little traffic as the brakes will need a lot of pedal pressure to stop you.

 

Intake manifold. Exhaust manifold makes a popping sound

 

Another vote for intake leak. Either the intake manifold or the TB gasket.

When you apply the throttle (throttle opens), manifold vacuum reduces. Once you back off and the throttle closes, manifold vacuum increases. The increased vacuum now pulls air in through the gap and whistles.

Mine did the same thing. Went away when the engine was warmed up. Guess the metal expanding helped to close the gap. Mine appeared after replacing a cracked exh manifold. At first I retorqued everything. Got better but was still there. Ended up replacing (again) the manifold and TB gaskets.

 

So I swapped the throttle body with the one from my other Jeep that I know is good and it still whistles. It's throttle specific not rpm specific, so if I'm coasting down the road at 1200 or 2200 rpm it'll whistle till I even barely touch the throttle. Could that still be the intake gasket? I'm going to go over all the vacuum lines again and see if there's a leak or crack I missed.

 

Check the engine vacuum with a gauge connected to a manifold vacuum source. I usually use the brake booster hose.

 

Yes, it is the gasket