noisy tappetts

As per the title really. Had these cars for years now and we recently had a few issues with engine, had the bottom end rebuilt, oil pump, and hydraulic tappetts replaced. The car drives like a dream but I have god awful rattle at certain revs, through the rev range mostly no rattle. I'm prepared to change them myself, how can I check them when exposed if they are operating properly or do just have to swap them?

Are you sure it's the tappets?
When mine are some times noisy when cold I can hardly hear them unless I have the window open, they usually quieten down after a few minutes or if I rev the car to above 2000 rpm then left the revs drop to idle but mine only tap on tick over.
I think the can check the gaps if you think there is something wrong with them, the Haynes manual tells you the gaps but no 100% sure you can do this with hydraulic tappets?  

I would check for anything else that could rattle, heat shield on exhaust etc.