If the "knock" is at 1/2 engine rpm (like 5/sec at idle) then it is
likely to be a waek hydraulic lifter (you don't need to adjust the
valves on these, either!).  It's hard to tell is a sound is serious
of not -- take it to somebody you trust to listen to it.

A weak lifter is not serious.  A runout big-end bearing, knocking
only when not cushioned with oil is a major problem -- engine rebuild
time.

My Vanagons sometimes do this, particularly if they've not been driven
for a week or so, and all the oil drains out of a lifter.  My Acura
Legend has done this for 100k miles.

malcolm