Interesting that your friends whines out of gear. My father in law's only whines in 1-4 and 6th. The assumption is because 5 is direct drive (1:1). So far mine isn't whining but who knows how long that will last. I am curious...of those w/ the whining problem, does it whine in all gears? Or all gears except 5th?
It depends on the source of the whine. Gear whine, bearing whine, then which gears or which bearings. Then if gears, why are they whining. The mesh error could be due to the case.
Now to complicate things further problem with gear noises is there are lots of normal gear noises. They will vary within tolerance ranges. Often there is no problem with the gears themselves, but rather something is transmitting and or amplifying the noise. There are a lot of noises we normally don't hear because work has been done such that we do not hear them. This is why putting in a new shifter can change the entire sound of the transmission. The noises were always there, just not transmitted or loud enough before.
Just small manufacturing differences can result in noise. Then again so can defective parts. Tearing down a trans
to find out is expensive.
Also, for the first time in decades Ford changes the source of the mustang transmission, so there's all sorts of new quirks that are common to the new brand but not the old one.
I wouldn't expect many people at the dealer level to grasp all of this let alone all customers.
What is the range, the distribution of "normal"? What is "broken"? Everyone is still learning.