In the old shipyards, a riveter may have been trapped between sheet steel
structures. To escape they would tap on the steel
plate to make other workers aware of their dilemma.
So maybe there was a "gremlin" locked inside the engine
and is now seeking escape and freedom.
I once had such a gremlin but it turned out to be rattling of adjacent steering cables
. But it had me worried for some time.
Might I humbly suggest that you check if the noise
has the same frequency as the engine RPM
or if it matches the belt frequency. i.e. use a White-out blob on the belt. Also on the harmonic balancer (that front wheel
that's connected directly to the crank shaft), the engine water pump
, the alternator
fan blades etc. Run the engine and see which one matches the tapping rate.
The wooden stick can also work
but the white out blob is probably safer.