I hear ya that happened to me but it was my rear differential this is like a whistle noise when driving on the highway mainly and it only whistles when I let off the gas...

With a vacuum leak, the sound gets louder when you let off the throttle. That's because the vacuum gets higher because the engine is sucking against a closed throttle.

When you get out to check it, try manually opening the throttle and letting off. You should hear it then.

Is this the throttle body gasket?

Ok I the sound lasted long enough for me to get and open the hood. It's a high pitch whistle sound when I let off the gas, it happens while driving on the highway, it will last a little while in city driving but eventually stop. Again it only makes noise when I let off the gas pedal. So I opened the hood and pushed on the air resonator box covering the throttle body and the noise would change. It stopped when I pushed on the box and would come back when I let off the air box. Does anyone have an idea what this could be?

You guys all know the 4.0 has a natural whistle to it that is completely normal. It's how the intake is made, it just always has a slight whistle.

Hopefully you guys are chasing something else down.

It's not a natural noise. It's very hight pitch and loud. It only happens when I'm driving on the highway and i let off the gas pedal. That is not natural. Do you have any VALUABLE advice?

maybe its got one of these installed lol

Really dude? Lol

I'm pretty sure it's your piston return springs. They tend to get to whistling when they wear out. Your going to want to repair this ASAP.

Where are the piston return springs?

jeepcherokee99, I believe that they are in the 'Big Mechanics Book of Gags and Jokes'.....

Lol you guys are awesome... Anyway I think I just figured out it doesn't happen if the heat isn't on.

I have a 04 GC and it does the same thing. It sounds like a tea kettle when you let off the gas pedal. The colder it is outside the louder the whistle. I took mine to jeep and the mechanic said its the intake gasket. Its a $18 part but $500 labor. He said its a easy swap just time consuming. You have to take off the intake manifold and exhaust headers to replace it and its a good time to replace the valve cover gasket too, they are prone to be busted on that side of the valve cover.

This is exactly what mine sounds like

So I heard another high pitch whistle. I usually run my heat on the auto settings. When on high I could hear a whistle inside the dash from the blowers in the cab. Any ideas what vacuum leak that could be.

I have the exact same thing going on with mine. I can literally play a song with my gas pedal. I've pinpointed the region of where its coming from in the motor but not sure exactly where or whats causing it. The intake gasket seems to be the most reasonable solution. I've been driving with it for about a year and its starting to annoy me now.

I thought maybe it was my exhaust manifold gasket or maybe whole in one of my pre-cats or something.

2001 XJ Sport

Ya I have It for awhile and getting sick of it as well. I also have figured out upon research it is probably the intake gasket which is not easy to fix. I was just wondering if the high pitch from the heat inside the cab is maybe a different vacuum leak...