Whistling noise

My 1998 Pontiac Grand Am has just started making what I can only describe as a whistling noise while in second gear. My first thought was a transmission issue (it’s an automatic.) I checked the fluid level and it is fine. There is no other problem with the shifting - it’s smooth and consistent. Does this definitely sound like a transmission problem or could it be something else? I’ve never heard this sound in any other car I’ve owned so I have nothing to go by.

Perhaps you have a vacuum leak. Maybe a loose or split hose?