Could be the clutch release bearing needs replacing,when idling does the noise change when clutch pedal down?
If the gearbox it self is playin up then ur best findin another gearbox with out crunch issues.
If it was lsd box then would of also knock on low speed on slight turn,wont be cheep to get rebuilt,
Had my oem standard box built cause was hard to find a decent box then,had lost 2nd,3rd and 5th gear cost me £350 cash at the time.
Now av gone oem lsd so just kept the other box as spare back up.
Heard some horror stories about second hand boxes ...thing is release bearing is only in it since Christmas ...it's a mad vibration but actually goes when u clutch it only when coming down high in the revs