Whats causing this noise? on 2003 Ford F-150 Lightning

My 2003 lightning has a high pitch whistling noise(sounds like a tea kettle) coming from behind the supercharger a little to the right. It only does it idling or when im at a steady speed but when I accelerate it stops. What could this be?

pcv elbow may have a vacuum leak causing this. common issue with these motors.

Thanks and if not replaced soon could it cause more of a problem?
it will throw a code eventually. i would get it confirmed asap.

where exactly is the pcv valve. is there a website i can go to?
It is on the passenger side valve cover. There should be a sticker with a diagram under the hood.
It was the elbow thanks but when I took the hose off the intake I noticed just a little oil, was the elbow causing that or is it something else
That is normal due to wear in the pcv valve just replace the valve
1 more answer
It sounds like a boost or vacuum leak