I've been getting a very high-pitched noise after the car warms up whenever I lift my foot off the gas, and it persists whether I'm in neutral or in gear. It's such a "pure" sound that I initially thought it was feedback through the stereo system, but opening the hood revealed to me that it was coming from the front of the engine. The pitch doesn't change much in relation to RPM, and it sometimes goes away in neutral. I tried loosening and tightening the supercharger belt tensioner to no effect. Has anyone else had an issue like this?

One of your other idler pulley bearings? When you are coasting, there is less tension on the belt? Also, try it with your supercharger belt off altogether. Car will be sluggish, but ok to drive.

Well, the sound doesn't seem to be coming from the front of the engine, but rather from the intake side. I'm suspecting a vacuum leak, seeing as how the pitch doesn't change very much with the engine revs. Are there any areas of the intake piping that are particularly leak-prone?

what about the vacuum pipe for the brake booster?

I tried fiddling with that, but it had no effect on the noise; same with the capped vacuum take-off that's right above that.

I hear the same thing on mine NA but it is coming from the fuel supply line going into the fuel pressure regulator,there is a 90 bend that I am thinking it has bent to far? It is plastic with a insulating cover on it that I have not cut yet to look closer at.

There is a small air hose on the fuel pressure regulator, from the intake manifold, has that got a leak?

It’s worth have a look for a cracked or lose vacuum hose for the supercharger bypass actuator. If not functional there is a very weird whistling or whining noise at coasting and idle.

Try unplugging second air pump, they start to wine.. Please advise.

Spot-on; I just heard back from the Jaguar whisperers at Samuels Jaguar Motors and that's exactly the problem.