It may be worth making a recording of the noise to help with diagnosis, but if you're stumped, I would recommend a call to Russell Walsh of
MG Rover Solutions.
My V6 had a knock from cold which disappeared when warm and stumped a good many experts. After 12 months of head scratching (and ££s spent), I contacted Russell at my brothers recommendation. Russell has some 40 yrs experience with MG / Rovers, and identified the noise in 2 minutes. The subsequent stripdown showed his diagnosis to be spot on, and as a bonus, his charges were more than reasonable and his work ethic 2nd to none
.
Cliff