From your efforts, it sounds like the fan was hanging up on that "crud". Just do a good cleaning to totally eliminate that "dull knocking sound". The fan runs fine outside the unit - so it has to be interference making the noise.
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=== Update (12/24/2022) ===
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That fan doesn't have a starter capacitor on it, does it?
From your efforts, it sounds like the fan was hanging up on that "crud". Just do a good cleaning to totally eliminate that "dull knocking sound". The fan runs fine outside the unit - so it has to be interference making the noise.