Whining noise on 2000 xj auto

I have a loud whining noise coming from the driverside rear section of the engine bay it happens after drivin the car for a few minutes and will stop as i go above 1/4 throttle and then will come back Please help!!!

there aren't many moving parts in that location/area. I'd look for a vacuum leak that comes and goes dependent on throttle position/total vacuum.

Intake manifold bolts can get lose
Vacuum brake booster diaphragm can develop a hole
Throttle body gasket can leak
vacuum fittings on intake can deteriorate and start leaking

Somebody snuck into the engine bay and installed a 6-71 blower on your 4.0 engine while you were sleeping!

As that is highly unlikely, I'd agree that a comprehensive vacuum leak search should be underway. Vacuum is much higher when the engine is at idle and that fits your description.

Know how to check for a vacuum leak?