LOL, no roll back, although I did some great ones in my Mum's Datsun 180B many years ago and broke nothing.
I dropped the car off at Revzone (who do all my work) this morning and they have just rung me to say the noise occurs in neutral as well so they will be dropping the diff for a proper inspection. I'm sure it's not leaked any oil and is running dry. Perhaps a bearing has failed?
Thanks for the feedback BGTV8 as it's all still very interesting to me.
