Just had a trans installed at ford dealer in overdrive trans whines hit overdrive switch noise goes away on and off @40mph
Auto Trans on 1995 Ford Mustang
This repair should be under warranty from the ford dealer, you should contact them before your warranty runs out.
For many repairs, Parts and labor at many dealers has a 1 year warranty attached to that repair. If the dealer has just replaced the transmission, and it's already whining, definitely call them
could be the gears in the rearend or the tailshaft bearing