[rotorwrench]

Solid tappets always make a little bit of noise which can get louder as the clearance increases. As the cam follower is pushed up and gains momentum, it will tick when it makes contact with the valve stem. The farther it has to move before making contact with the valve stem the louder the tick. When I hear a loud tick, I generally start looking for a hole in an exhaust pipe or a burned valve but adjustable tappets can come loose and open up where non-adjustables won't.

When a valve is opening or closing it is always under spring pressure so it it loaded against the valve train. They generally won't make noise when they are loaded under spring tension. If the valve sticks then it is a different story. It can make noise then or it will stick and just stay that way.