Diagnostic Advice

I recently purchased a KHS Flite 780 off Facebook Marketplace and I frequently hear a click-click-click sound during a pedal stroke. Not all the time (depends on the intensity of the pedal stroke) but enough for me to start a thread to ask for advice.
The sound doesn't seem to have an overall negative effect, but it's kind of annoying. Can anyone let me know how to possibly figure out the problem without spending money? I can't tell if the sound comes from the pedal, crank arm, or bottom bracket.
Thanks in advance for any advice.
If you have the tools and time, the best would be to remove, clean, and lube all those parts.
Pedals, crank arm attachment to BB spindle, BB bearings.
Simple first check would be to verify tightness of the BB retainers, and crank arm retainers.
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wheel skewer, seat post clamp, headset

I had a click one time and tried every 'obvious fix', it turned out to be the front wheel skewer (not what you would expect with pedal induced noise, right).

I had a crackle under hard seated efforts and tried every 'obvious fix', it turned out to be a little lump of paint under the seat post clamp.
-a dry seat pots/tube interface can also cause 'drivetrain clicks'.

Never overlook the headset for noises, even ones you think are in the drivetrain.