Yup... let me guess, you put on a Weber carb,... right???
If so, I think you tightened your carb-mounting bolts too tight, and are prying the adapter plates apart just far enough to create a vacuum leak... just loosen your mounting bolts a tad (maybe 1/4-1/2 a turn) should take care of it.
Actually, I just did a highway trip and when I went over 3,000 rpm I got a slight whistle/high pitched rushing air kinda noise. I just kept it under 3,000. Sounded like it was coming from the air cleaner (yes, I've got a K&N).