The noise does not seem to be coming from under the hood....more interior.

 

A whistle when you let off the throttle sounds like classic intake manifold leak to me, have you checked all the bolts? Warm it up real good and tighten them. Ive had this happen on several jeeps ive worked on. Also with it running, spray carb cleaner around intake gasket and listed for the engine to bog, theres your leak.

 

I will try the carb cleaner and see what happens.

 

I've had those big washers on the intake/exhaust manifold crack and fall off, causing a leak there and making a whistling noise, only when you let off the throttle.

 

My xj is still doing this and it is definitely not coming from under the hood. I can pop the hood and push the throttle a few times with my hand and don't hear the whistle/chirp.
But you can hear it inside. Does anyone know the location of any interior vacuum lines? I believe that this is the issue

 

OK guys I have finally had the time to track down the noise. I removed the glove box and found a harness of vacuum lines. The vacuum line that is whistling, is the green vacuum line going to the air diverted/flap that's under the passenger side dash. This is what moves the air flow from defrost to floor etc. If I remove the vacuum line it automatically goes to defrost. Does anyon
e know of a place where I can get a replacement line so that I can get rid of this whistle?

 

Anyone got any tips or where I can get parts?

 

I have the same thing coming from the drivers side where you able to figure it out? All my heat settings work

 

Gonna have to poke around under there.