I do not know if this is the whistle you are hearing but if you have the stock air box i think when it is sucking in the air it gives off a little whistle??
98.5 Black SVT with 34k miles Build #5683 Born on 4\15\1998
WR Headers GMK CAI
Blaupunkt system fiberglass tweeter pods
L.E.D. tail lights in the works