Do you think it might be something that a synthetic fluid could quiet? Amsoil is popular for any Mustang transmission component. I also add a few oz.'s of Militec1 to all my fluids in the car. Please update when resolved.

 

i had to change my tranny gasket because it was leaking, after doing this AND FILLING IT BACK UP THE NOISE HAS WENT AWAY, it comes back very lightly once in a while but only for a second or two. Go figure

 

Good. I use Mobil1 automatic transmission fluid. I use Amsoil 90weight for the differential.
I was told to keep the "camshaft positioning sensor/synchronizer" in the back of my mind because it is the cause of the noise. So I am passing that word of advice on to you too.