Originally Posted by macgyver35

Generally (not always) vacuum-related leaks which cause a whistling are more pronounced at closed throttle when the engine is making the most vacuum. There is a fairly common "tea kettle" whistle that many get from intake manifold leaks, but again, it usually shows up at idle. In my case, it showed up precisely at the moment I pulled up to the drive-thru window at McDonald's to order lunch. Loud as hell and quite embarrassing.

That being said, it's almost surely vacuum related, even if it's not at idle. XJ's make a symphony of various noises. Squeaking, rumbling, grinding, flapping, ticking, etc. But whistling is almost always vacuum related.

Originally Posted by xj_maniac_newb

Has it made this noise since you got it? ..Do you have a throttle body spacer?