2008 A6

I have a new sound coming from my car only at part light throttle only... It sounds like something spinning then it stops vibration under my feet.. Kiinda like a flat fan that you would push down table... fast metallic buzz noise that slows down and eventually stops at WOT... I drove an 2 hours today and made a few passes at the track and it's holding up... I have a Vig 3200 stall... It doesn't make the noise in park or neutral when I rev it... Any Ideas.. A buddy told me it might need more tranny fluid..? Any thing else?