Have you tried taking the cover off the impeller to see if something's jamming it up? If you're having a lot of deposits elsewhere, it's entirely possible that they are gumming up the pump impeller, too-that would cause the hum at first and would explain the noise and why it's running hotter than usual.
Just a suggestion, but it's the first thing I would check. Make sure that the power is completely off to the motor before taking that cover off!!
Janet
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