Can't tell in video but do those nipples have a union for easy removal?
I'm thinking you have a ball or rubber type heat trap with sediment build-up that could possibly be causing the noise. Just a guess.
Unfortunately, the only way to know for sure is to remove the nipple. With a water heater that's over 15 years old, flushing sediment, old valves, removing nipples, etc., could be opening a can of worms.
As you found out when you opened and closed the wh drain valve, sediment got stuck and valve didn't close all the way causing it to drip now.
Please wait for the plumbers to respond, they will have more suggestions/advice. Thanks.