Gearbox issue

[speed_nut_yn]

Hi all

I have come accross an issue with my car. Its a 52 Bora 1.9 TDI 130. I can hear a loud knocking / tapping sound when driving along in 4th or 6th gear (other gears are fine), does this mean the gearbox is gone?

If i need to replace the gearbox whats the best / cheapest option.. 2nd hand replacement or a refurbished box, and what kind of price would i be looking at?

Lastly the gearbox code on my car is ERF, If i replace it will i have to get one with the same code? Can anyone recommend anywhere to buy a second hand box?

Thanks in advance