I haven't watched the video yet, but it is possible your knocking noise is your PTO, as a couple others have mentioned. they have anti-rattle springs that often fall out or are forgotten or eliminated by previous owners. testing this is super easy. with the engine off, engage your pto (lever forward) and grab the pto pulley and turn it back and forth. if it has some wiggle room, there's your knock! make sure you've got your key in your pocket so you dont accidentally leave it in the on position when you try this. Glad you brought your 125 back to life, they're one great little machine. 
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Ian
Mine: 72 w/ k301, 3 125's, 1 w/ hydro lift, 38,42, & 2 48 decks, 2 42 front blades, QA-36a & QA-42a thrower, tiller, lo-boy 154. Also, LOTS of parts.
family's:2 105's, 2 106's, 149, 2 lo-boy 154s, Farmall Cub.
Non-IH: 1940 JD Model A, 1954 JD 40 U, 1955 JD 40 Crawler, 2 1956 JD 420 U's, both w/ Henry Loader and Backhoe. JD 110. Wheel Horse (model unknown.) Power King 1614. We love our tractors! 
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