Due to the frequency of the noise, my only other thought is that it could be little ends. Not sure how to check this in situ though. Think the pistons will need to come out if this is the case. Either way, timing chain is going to be first port of call, them sump off for and investigate. If all looks good there, the head will be coming off next. I think I fitted a composite gasket, so not looking forward to scraping all the shot off and keeping everything clean...