Just from reading your subject title, I was going to suggest your windshield wipers. Some windshield wipers have holes in the mounting brackets, and while driving they tend to make a whistling sound...
So, with that NOT being the issue...
Take a few minutes out of your day if possible, and stop by a friendly neighborhood mechanic, pop the hood, and see what you guys can come up with.
My next suggestion, would also be the belt tensioner.
Keep us updated on your problem, and if you're able to solve it.
2006 GTO, Cyclone Grey
Mods: Magnaflow Exhaust