How do I balance an attic fan?
A few suggestions. 

None of these include removing material from the blades (because after doing so you would indeed have a balance problem).


Forgive this one, but are you certain the blades are installed correctly? The fan indeed exhausts, when it's running?
In my experience, "replacement" motors, even under identical part numbers, might be subtly different from their originals. If this new one is slightly shallower, then you've done nothing wrong. Maybe check them against each other, and then cut yourself some slack.


(The fact that your fan isn't mounted dead vertically could cause you to have a problem that the manufacturer's quick &amp; dirty testing failed to detect. Because gravity.)


Those bent-sheet brackets are a primitive thing. They're likely to have gone asymmetrical. Maybe put the palm of your hand against the motor's base, and press firmly to bend them, just a little.
Unless, if I understand you, the position on the blade assembly on the motor shaft is indeed adjustable. If so, then why did you bother with the washers? Side it on out!


None of the above explains why the problem didn't appear when you tested it. Only with a hot attic. Difference in air density? Behavior of the motor's lubrication? Or (as already suggested) a loose mounting screw?

Good luck. Let us know what works for you!