I've read through several threads on this and know that maybe I'll never find the culprit.
this is the noise
it's not constant. But happens when I ease off pressure pedaling and then start again. it does it when I'm standing up as well as sitting. It doesn't do it when I flip the bike over and pedal. Feel like/sounds like a click coming from either the freewheel (an old shimano, brass I think?) or the crank/bottom bracket. It almost like I can "feel" a little give with each click in the cranks. When I pull/push side to side away from the bike there's no play in the crank arms. My local bike shop did just adjust the rear derailleur and put new pedal cages on - it wasn't making the sound before. I don't think they would have touched the bottom bracket tho. Thanks in advance for any tips!
It weirdly comes and goes, but when it's active like in the video it's super annoying. But I spent 10 minutes trying to get the video - while it wasn't making the sound

Twice per rev and click in the same place as you pedal?
- loose crank (check for play in rotation, not just side-to-side)
- big pit in bb cup or spindle
- loose pedal
- some part of the toe-strap hitting
- (worst case?) broken frame

IMO, You have a problem with the rear derailleur and/or chain, or maybe the cassette lock ring. Have you tried using the derailleur limit screws to line up the chain with the dr pulley wheels and cassette. They should be in-line. Have you checked the chains length or the quick connect link? Have you removed the cassette and maybe replaced it with the wrong lock ring? You can see the chain skipping a bit when the clunk is make. Another possibility, have you replaced chain, if so, is it he proper speed for your components.
You will find the reason, it is a matter of elimination. Try one thing at a time, and know what you did so it can be reversed if that is not it.

Given the derailleur adjustment, it sounds a little like the chain is getting snagged on the next cog up occasionally. What gears does this happen in?

Yes,the linked video clip sounds like a derailleur adjustment is needed. (as another comment mentioned, check for a loose lockring first -- can you wiggle a cog from side to side at all? sometimes it's due to a missing spacer behind the cassette on certain hubs.)

If the chain is too far from the center of a rear cog, it briefly catches on the next larger cog (or next smallest in the other direction.)
I use a dollar bill as a test: can I slide it between the chain and the next cog without the paper binding? Check both sides of the chain, larger and smaller cog. No binding? That's a reasonably good adjustment for the rear derailleur.

I bought a once bike that made a similar noise. I checked everything except the bottom bracket because I didn't have the right tool. When I finally took the bottom bracket apart, the bearings on one side were severely pitted. One bearing looked like a half-eaten apple. Luckily, there was no damage to the cups or spindle. The prior owner had stored the bike outside, leaning against a shed, so rain water had travelled down the seat tube and accumulated in the bottom bracket on one side. The stuck seat post should have been my first clue to check everything else a little closer.

Edit: I should have watched the video first before responding. That sounds like it is coming from the drivetrain and resonating through the frame. I would check the pedals, chainring bolts, and crank bolts. Does the noise occur on both chain rings, or only the large one?

Do you have friction, or indexed shifting? It does look and sound like the chain is catching on the next cog, or a bent tooth.

Check that all the rivets on the chain are positioned right. If you are using a quick link, check that it matches the chain. Using a 8 speed link on a 9 or 10 speed chain can be induce, what some might say, random clicking.

I would not be surprised if the cause turns out to be a stretched chain.

Thank you for all the replies and suggestions - Lots to investigate here.
the click isn't always in same place but usually is...The chain was replaced recently but wasn't doing this until the recent rear dr adjustment. it seems to happen in the second and 3rd smallest rear cogs but I need to confirm this again. It's friction shifting and only seems to happen on large front chainring.

What gears does it happen in?

Easiest troubleshooting is to replace things; start with the pedals.

It really sounds like the problem is with the lock ring, problem with a quick link or frozen link, or a stretched chain/worn out cogs. When I look at the movement of the chain, it looks like it is getting hung up on the either the cogs or the pulley wheels. Is the chain properly installed through the derailleur? What did you do to the drive train before putting it back together? Being that you state it is friction shifting, I believe that means the problem is more likely with the chain and the cogs.Maybe you should take it back to who worked on it.

Work stands are really helpful for this kind of thing.

The quality of the sound to me seems like an out of tune RD or otherwise skipping cog. I assume you’ve tried micro adjustments with your friction shifter, but if not try to “tune the noise out” with the shifter. If that works you have your answer. If not, maybe look at a stiff master link in new chain or worn freewheel cog. Another issue I find can pop up is with under BB cable routing, the cable pops out of the guide, which can cause weird shifting.

My guess is a broken dog or spring acting upon the dog in the freewheel, where now only one instead of both are catching. This spring had an end broken and caused the same type noise and eventually enough freewheel drag that it would upset the chain's free movement.

update: took it back to who worked on it: they first adjusted something on the chain, not sure it was the masterlink, but that seemed to help a lot.
It was still doing it, even tho less noticeable, so I had them put on a new sunrace 6 speed freewheel to replace the old shimano 5 speed.

That seemed to help even more. I'm still getting a slight version of the noise when I'm in the second highest gear (big ring front, 2nd smallest back) which leads me to suspect that it may be something with the bottom bracket. But I'm trying to do my best to just ride the bike and stop obsessing over it.

It still makes the noise.
https://drive.google.com/drive/folde...usp=drive_link

Against the advice of nearly everyone, (on here and in person) I changed the bottom bracket from a steel stronglight to a titanium stronglight. I also replaced the pedals.
So it's not either of those two things. The chain is only 1.5 years old, and the freewheel is only 1 year old. A cheap sunrace one. When I remove the wheel I can feel some play on the freewheel side to side, when the bike is upside down (I have no stand) and I pedal and then let the wheel spin freely you can see some "wobble" to the freewheel.

But again, it made the sound before the sunrace freewheel was changed. I'm tempted to try changing the chain. Or try another freewheel.
I'm also slightly suspicious of the rear derailleur, it's never shifted very smoothly. If money weren't an object I'd get a dura ace 7400 rear derailleur and matching shifters and freewheel and switch to index. Will maybe do that anyway.

I'd really like to figure out what the sound is though. Could it be possible that my Tipo hub has some sort of issue, like the bearings? Could it be the skewer, which is not the original campy one but a cheapo anti theft one? Would welcome any thoughts.