I agree with Nick above. First blush guess without being able to see/hear/ mess with it in person is rod bearing. Could be wrong and a car stethoscope or long screwdriver as a proxy is a good way to probe around and point to actual location
Stethoscope on the intake and around there, and on the oil pan for the rod bearing. I would have thought a rod bearing would be constant. My initial observation was on a client car. Mine never spun, but I could hear a ticking from the passenger foot well under acceleration only.
Had the car been sitting unused for a long time? Rust could have built up on the cylinder walls. Check the compression. If low, driving it around for ten minutes will scrape all the rust off and restore the compression. Rings may also be gummed up.
This and the one you had in the other post has me itching for warm weather and tearing into them to see what the rotating parts and cylinder walls look like. Of course 5 hrs in with my back aching would change my mind. Watching your project
Sounds a little too loud to be spark arcing, but are all the spark plug wires firmly on the plugs?
Pull the oil filter and cut it open and see if there's metal in it. If not, I'd probably consider pulling the valve covers and look at the condition of the chain tensioners and the cam lobes and shims. If a shim has left the bucket it'll sound kind of like a knock.