BKD Gearbox Wine

Hi all ive just purchased a 06 plate Leon with the BKD engine as a bit of a project, the engine is strong and been well maintained but the geabrox is giving me some cause for concern. when in 5th and off the throttle there is a wine between 35 and 60 mph, no other gears make this noise but i fear it's gear on the shaft.

is there a better box to mate with the BKD engine as an upgrade as i'll be planning on putting more power and torque through it but not until i get this issue sorted

thanks in advance for any replies