R52 gearbox

I have just bought an 05 cabriolet which is making a loud and low ticking noise that increases with speed not revs.
I assume it is a getrag box which has gone but wonder if it might just be clutch related.
There is no clutch slip and all gears go in and stay in ok except reverse which grinds when entering gear.

Is there a way to diagnose if it’s the box or a clutch component ?

if it is the box what is the cheapest way to fix it ? 2nd hand 5 speed getrags look a lot more money than 6 speed ones but 6 speeds look like a pig to fit with all the related components.

Any advice appreciated


thanks