Intermitant noise when changing gear - C5 RS 6

[geordieRS6]

Hey, been a while since my last problem. All has been running well, unusually. But, here we go with another issue. Its a tricky one to explain, but bear with......
As stated in the header, this is totally random. The noise happens when changing up. The best way I can describe it is like a grinding noise, but it goes up n down with the revs. It is totally intermitant. I can go days without any issues. Then, suddenly the noise starts. I can then stop the car, turn off engine for a few minutes, start again and the noise isnt there!!