on my 72, the pto clutch disk "teeth" (the pieces that stick out into the basket pulley to drive it) are rather worn down and the anti-rattle springs are all long since departed. that creates my knocking sound. to test, with the engine not running, engage the PTO and try turning the PTO pulley by hand. if you get a good amount of "wiggle room," that could be your knocking sound.
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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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