My DB has a replacement A112 with 70hp. It was raced in the 70s and 80s.
The gearbox is a 600D but with 11/39 (3..54) final drive. I had it installed to increase top end speed which was lacking with the existing 9/39 (4.33) final drive.
I have noticed whining in 3rd and 4th gears above 4000 rpm.
Of course, knowing these boxes were designed for much less HP and torque, I am wondering if this would be an expected result. I do drive with this in mind. Not agressive starts, or maximum acceleration.