Two mad things I have had happen that might be worth checking.

Turbo mount siezed and I was convinced from the buzzing it was from the gearstick. If it were not for a local expert Geoff Dewitt I would never have made that connection.

Even wierder - and took about a year to suss out... starter motor came ever so slightly loose and although it started the car fine enough the shaft was bending a little more each time and making a buzzing racket. Again I thought it was the box.

You never know with cossies!!!