Is it up for debate that Teslas are generally noisier than their ICE equivalents at similar (and often lower) price points? I thought that was WELL publicized on this very forum (and for me, in real life).
The Model 3 forum is rife with posts. As an example:
My biggest improvement with road noise
Wind/road noise from doors/windows -- fix it for good
Noise reduction rear areas
Sound Deadening Model 3
The noise at freeway speeds for most modern cars is tire and wind related...not engine. Indeed, when you let off the gas on a modern car and "coast" the fuel actually stops flowing through the injectors I believe and the engine essentially "pumps" but nothing is firing (which is why you see those "instant mileage" trip computer readouts go to infinity on gas cars).
As to the upcoming Mustang, you are correct, I haven't driven it so I obviously don't know it it will be quieter, but I suspect Ford won't have suddenly forgotten how to insulate a car, weatherstrip a windshield or put better door seals in place just because it is electric.