Both?

and they both run awesome

But the comments also show how...if it ain't turning, there is no way the noise is coming from the valve train...

Sounds like exhaust too me too. That's probably really cold water coming out of your hose, and it's popping in the exhaust. You could try pulling the SWP belt off, and running them for a minute to see if the same thing happens. But an automotive stethoscope will tell you exactly where it's coming from. You can get them on Amazon or at Harbor Freight.

Also, you don't need to start them weekly when not in use. This debate goes on since the dawn of internal combustion engines. But it's best to just let them sit. Starting them weekly, you are putting them through dry startup wear WAY more often then needed...
...just to lubricate the parts. IF you want to do anything just to relieve valve spring pressure, bump the starter once every week or so just to rotate the top half to a different position.

Never heard anything like that. Other than hail on a metal roof. Weird.

Does it do it if you only let run for 10 or 15 seconds when it’s cold?

doesn't matter how long you run it..2 minutes shut it off does it...let it get hot still does it.

I have to mention my original post that I added an engine treatment for cleaning lifters etc. The noise got louder after putting this stuff in.. It was never that pronounced.
I ordered a set of new compcam lifters...nothing to lose at this point. If noise stops i'll do the other engine also. How can a dropet of water in a cold exhaust and cold to touch manifold and stainless pipe make a pop like that. Water hitting something hot I would think would make a hissing that steam boiling off

I feel something happened to the lifters from sitting. Didn't do this back when i put it away for the winter 2019.

To me the valve train sounds noisy in the first video, if it got worse after the slick 50 was added get that chit out and do a fresh oil change

Aluminum heads?

Washers under head bolts?
Pull some bolts and see if washers are cracked.

Well, judging from the sound and how it tapers off about 10 seconds after shutdown, I would tend to think it is caused by oil pressure bleeding down as the engine stops turning causing something inside the valvetrain to move and/ or creek. Being that changing the rocker arms fixed it years ago, I would suspect the cure was because of a change in lifter pre- load and not necessarily a change in rocker arm design. My approach at diagnosing this would be to run the engine with the valve covers off and see if you can pinpoint the noise. Use a mechanics stethoscope if necessary and also keep an eye out for any valvetrain motion once the engine comes to a stop . If that doesn't give you any more information, I would then pull the distributor out and spin the oil pump using a priming shaft to pressurize the engine with oil without the engine rotating and see if you could reproduce the noise. It is definitely an odd situation, in one that I have not come across before.