Originally Posted by tabetha

It could be a lazy tappet, but you'd normally hear it, as one being distinctly noisy compared with the rest, they can be taken apart and cleaned out easy though, I did on mine(just rebuilt engine after 200K), mine even when left 2 weeks is 100% silent on start up, not even the faintest hint of rattle, the tappets have done around 100K.
tabetha