I've seen this happen on a couple of different engines so far (BMW S62, Mitsubishi 4G63) but am not sure about how commonplace it is.
If an engine isn't used for an extended period of time, the subsequent startup results in a tapping sound that changes frequency with engine RPM. This sound then goes away after the engine is run for some time.
People usually associate this tapping sound with stuck valve lifters. I would like to better understand what causes the valve lifters to get "stuck" and what causes them to free up again without external intervention.