It has been said in other threads that most (i wont go as far to say all) epoxys are not waterproof, and are in fact, porous. Which is no good for a long term "relationship" with a pump . Use a silicon based sealant , goop, but not epoxy.
Point two is that usually the pump sounds is due to vibrations of the shaft or cavitation (lotsa air in the system).
Then two things : the shaft has head that is prone to vibrate , like the Eheim shafts, then someone developed
this mod , to give an elastic solution.
Or the shaft is misaligned . However, and i've never done this, but i think correcting an misaligned shaft should not be and easy task.