Mine did the same a few weeks back just after i got it on the road, it was almost a knock rather than a tap, i was using 20/40 oil
I put some engine flush in got it very hot drained it, new oil and filter with 10/40 semi synthetic and it purrs perfectly now
i played with cvhs alot a few years back when i 1st passed my test and they do seem to do this when they have been stood a long time, sometimes some flush and fresh oil and a run out sorts it, sometimes it dont. Some people say if its goona be stood a while to take the tappets out and store them in a bath of oil.
If you do need to change the tappets and want to do the stem seals whilst its in bits i have a tool to do the seals without taking the head off, giz a shout and you can borrow it if you like, i may even give you a hand!