I bought the car at something over 260k miles. If I remember, it occasionally was tappy like this on a cold start. I think these videos are cold/warm starts. I'll post a hot start next.
I changed the spark plugs and had to tap one of the holes, cylinder 4. Some dirt and shavings got in there. I may have been over-handling the rusty exhaust during the procedure.
After this, the noise seemed more noticeable (louder) and persistent.
I've since fixed the hole and checked for blowby - there is none that I can feel.
I compression tested each cylinder, from 1 to 4: 160 psi, 150 psi, 150 psi, 140 psi.
I did notice exhaust with my hand coming out where the header meets the downpipe. I've replaced a missing nut there, tightened everything up, and slathered a good amount of exhaust sealer around the mating area. I no longer feel the exhaust coming out there.
One of the studs where the header meets the exhaust port for cylinder 4 practically fell out on it's own. Still the header seems pretty solid on the mating area with the head, and I haven't been able to feel any blow-by where the header does meet the cylinder head, but the air being blown by the fan may confuse checking for exhaust here. There's probably a good deal of self-sealing rust, lol. I've been thinking about slathering exhaust seal all over these areas.
The sound seems to me to coming out of the cylinder 2 area.
I recently checked the clearance of the adjusting discs/shims and here are the results:
Cylinder 1, exhaust: 0.014"
Cylinder 1, intake: 0.013"
Cylinder 2, exhaust: 0.016"
Cylinder 2, intake: 0.012"
Cylinder 3, exhaust: 0.016"
Cylinder 3, intake: 0.015"
Cylinder 4, exhaust: 0.016"
Cylinder 4, intake: 0.014"
All of these seem to be within spec, according to the Mitchell data I could get my hands on.
None of the tappets seemed unusual in appearance or movement while doing the clearance check, but I haven't taken the camshaft off yet to pull out the tappets to see what's going on in there.
Despite what I've done (sealing certain manifold areas, etc.), the sound has not been affected or changed in any way.
Piston slap or rod knock of course has been in the back of my mind this whole time, but a few other experienced wrenchers and technicians seem to say lifter/valves/tappets when they hear it in person because the sound does seem to coming from a little higher up.
The one thing I haven't done is changed the oil since the plug hole tap. I was wondering if that might help... put in fresh 10w-40, or perhaps even something thicker?