Here is the sound sample cleaned of 90% of the normal engine noise, with mostly just the tapping left.
Gotta say ... if the tap is crank speed the engine was running at 320 rpm ... which I don't think is likely.
If the tap is at half crank speed .... rpm 640 ... far more likely.
I'm going with something turning or firing at half crank speed.
This sample is running at 50% of real speed.
https://youtu.be/44IBBGRU3VM