I recently bought a 2000 XJ and when its running it makes a loud squeal/whistle, it randomly stops and starts again, it also gets louder when i rev it and it sometimes goes away. If any knows or has experience dealing with the squeal/whistle id greatly appreciate the help

 

You have one or more of:
An accessory with a bad bearing
A bad belt
A loose belt

Go get a Continental elite "gatorback" belt. They're awesome.
When you take the belt off, rotate each pulley by hand and see if any exhibit any noise or have any roughness in spinning. Commonly the idler pulley will fail. It's a cheap and easy to replace part - enough so that you might consider just replacing it with the belt.
If none of the pulleys feel rough or make any noise, put the new belt on and make sure it's tightened to spec. A proper tool is best and the 00-01's are known to be super picky about proper belt tension.

If you don't want to spend any money - at least get a belt deflection measuring tool and tighten the belt to spec and see if it helps or affects the noise.

 

If the sound occurs only when the heater is on an A/C setting or defrost setting, it may be a bad A/C clutch or compressor failure.

 

This is a link to what the noise sounds like
https://youtu.be/0DQxAVFVCdk

 

It was a broke upper intake manifold