Gear box noise at 55?

[Treadlox]

Hi, my surf seems to produce a noise when it reaches 50~55 mph, a whine comes into play after it goes into top gear and stays there until the speed drops and then next lower gear is selected. Its an auto by the way.

It selects gears sweetly drives well and no loss of power. Manual changes are fine and the clutch seems strong. Gear box oil is clean and at the correct level.

Any advice is appreciated.

Cheers, Tony.