Ok so I think i already know the answer to this and I'm just in denial, but just so I'm 100% sure I thought I'd run it by you guys.
During the past weekend I embarked on a roadtrip to the 24hr Le Mans in my Mini. On the way home I began to notice a noise which I haven't heard before. Whenever I put my foot down in 4th gear I could hear a rough gravelly/grumbley sound. This only happens in 4th gear and only under load. If i dip the clutch in 4th and blip the throttle there is no noise, but it'll resume as soon as it's back under load. I can still hear it when going downhill but it's much quieter.
So... Is my gearbox knackered? I was planning on taking the car off the road for a couple years after this summer anyway, so will it last a few more months? To say I'm annoyed is an understatement as this is a relatively fresh engine/gearbox build and only has maybe 2500 miles on it.