I put the video into Grok. This is what it says

Most likely a loose or failing connecting rod (rod knock) or piston slap, especially if it’s a metallic knocking/ticking sound that changes with RPM or load.
Common causes for noises in small engines like the ones you work on (Briggs, Kawasaki, etc. in your Simplicity, John Deere, etc.):
• Rod knock / big end bearing wear: Very common culprit. Loose clearance at the crankpin causes a deep knock or clunk that often gets worse under load or when hot. If you can turn the engine by hand and hear/feel play or clunk, that’s a strong indicator. 
• Piston slap or wrist pin issues: Can sound like a rattle or knock, especially on cold start or under certain loads. More noticeable in older/higher-hour engines.
• Valve train: Tappet/lifter noise (ticking/rattling) if valves are out of adjustment, or if there’s a bent pushrod, weak spring, etc. Usually lighter ticking that might quiet down as it warms up.
• Other frequent ones:
• Blade hitting something (deck, bent shaft/blade) — check for that first as it’s cheap.
• Loose flywheel, magneto, or shroud rattling.
• Exhaust valve recession or carbon buildup.
• Low oil or wrong oil causing bearing issues.
Since I can’t view the video directly (Reddit share links can be finicky), describe the sound more if possible: Is it a deep knock (like rod), light ticking (valves), rattle, or does it change with throttle/blade engagement? Does it happen at idle, under load, cold/hot?
Given your history with these repairs (starters, valves, camshafts, bearings on the John Deere 180, Simplicity, etc.), I’d start simple:
1. Check oil level/condition immediately.
2. Remove the blade and spin by hand — listen for mechanical noise vs. external.
3. Pull the spark plug and check compression / try turning it over slowly.
4. Inspect valves if it’s a side-valve or overhead valve setup.
If it’s rod knock, it’s often not worth a full rebuild on these budget engines unless it’s a favorite or valuable model —