I think I might have the same sound on my 85GT 4sp. Happens mostly in 2nd but at times in 3d. To me it's a sound like a piece of thin metal vibrating from air or vacuum moving it. Also sounds like it's under the car. Only latst for a second or so.
Have not found mine either. Have been thinking of disconnecting the vac line to the brake booster to see if that would make any difference.
I've think I've eliminated clutch or TO.
[This message has been edited by Dodgerunner (edited 11-01-2007).]