Piston pin bad ?

^^ Yeah, it sounds like a valve train noise from something binding and being slapped loose, not a piston or rod/crank. I would check the valve stems, guides, etc., very carefully. You may have some gum formed in a guide or 2 causing a valve to jam open. It'll sound like your recording.



Ths picture of number 8 piston top looks like simple moisture corrosion; did you say this has been in storage for 5-7 years? If so, one or more cylinders will have it's valves open all the time, and moisture will get in. Happens commonly on small aircraft engines that sit idle for some time. You fill the cylinders with special oil for storage to prevent the corrosion. (easy to drian on small aricraft engines, hard to remove from car cylinders....) If you get the pistons out, look around the number 8 rings to see if there is a little bit of rust on them or in the groove and for pitting on the cylinder walls.