I inherited my '91 from my dad, who left me lots of opportunity to get it back to a reliable status.

I'll start with the most pressing issue first. It recently failed smog, after which I found an un-nutted exhaust stud (try saying that 5 times fast). I also ran some Sea Foam through the brake booster vacuum line and tightened up a belt that drives the alternator. Ever since that work, it makes this noise:

Youtube

I don't know what is wrong. I first I though bent or broken valve, but it seems to idle too well for that. It went from some minor valve tick to this box of rocks.

Ideas? Thoughts?

unfortunately i could tell you without a shadow of a doubt where= that sound is if i could get my hands on the car. if you can get a stethoscope and put it on (non moving) parts in the engine bay that will help you pinpoint the problem. i would check the alternator belt. set it to the correct tension not just tightening it up. you can snap belt like that real quick. my guess is it is the alternator or water pump pulley. as for the smog testing try nice new exhaust studs, nuts, 2g manifold, and maybe replacing other gaskets that are leaking, and as well as checking operation of the egr and pcv valve. is the check engine light on? has it had a tuneup? are there any boost leaks? you need to cover all the bases before you go crazy and replace the ECU or the catalytic converter. make sure the engine has everything it needs to run right then do the smog test.

Sounds like something going on under the valve cover. I'd try to narrow it down to make sure, then if so, pull the cover and take a look around.
Hard to tell from a video, but could be some extreme lifter tick or maybe a rocker that has come loose possibly due to a bad lifter.

A set of revised lifters is always a good idea in these cars to quite a noisy valve train. May solve your problem.

i agree with ryan, pull the valve cover and check in there. sounds like a few dead lifter having the rockers thrown around to me, that or the hydraulic timing tensioner is starting to go bad.

I think in my meat headedness, I overtightened the alternator belt and f'ed up the water pump bearings. I'm going to get a stethoscope and try and verify that. I think my smog issues were due to the exhaust leak, and so that is not the focus on my efforts right now.

This sound happened all of the sudden, and this is one thing that encourages me to think isn't the hydraulic lifters doing.