I have a 98 xj sport and I've never had this problem until now, it sometimes comes and goes but when I start my truck there is a loud whistling almost like a vacuum leak noise by the heater core it sometimes gets louder under acceleration and sometimes just at idle it's extremely loud any suggestions what this could be?

 

Have you checked for vacuum leaks? You are the only one that will be able to say where it's coming from since we are not there listening to it.

 

Are there any vaccuum hose over in the heater core area? I think there are ones for the blend doors and also it's a vaccuum leak sound because I hear I when I'm outside of the truck to its coming from the heater core area

 

Plastic line that runs to the bumper ball.

 

Checked and that's fine to I'm going to check my pulleys I hear a lot of noise coming from the engine bay I think it might even be my new alternator that went swimming last time I went offroad haha

 

Replace the CPS and TPS.

 

Not sure if this will cure the OP's "loud whistling noise", CC...

(although good advice if it was relating to the swimaction he had recently)

 

Isn't replacing the CPS and TPS the fix-all for XJ's. LOL

 

Very likely!
And if not....well, than it will have fixed something before it broke anyway.

"Eventually is will always be the CPS"....(much like in evidence based medicine: death being the Mother of all end-points hehe)

 

Would the CPS make this type of noise?

 

Just kidding about the CPS and TPS.

You may want to get a 2-3 foot piece of 1/2" +/- inside diameter hose that you can hold to your ear and poke around suspectd areas in the engine bay and under the dash when the noise occurs and see if you can pinpoint the source.