Hum after rear-end gear change
It IS normal. Don’t listen to anyone that’s says different and to find a new mechanic. If you go to a beefier set up or different gears, like racing gears, it will whine much louder. Try auto cross or drag racing with a differential and gears set up for that. It’s as loud as can be depending on the gear ratio. Especially in a coupe that doesn’t have very good sound dampening.