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i wouldn't be so quick to dismiss their reasons. while it does sound unlikely (ie: why don't other powerheads demagnetize) it's entirely possible. 'permanent' isn't an absolute when talking about magnets, it's just to distinguish from temporary magnets like electromagnets. things can be magnetized, and demagnetized. heat and strong magnetic fields can do this. possibly the startup uses a stronger magnetic field than normal powerheads we're used to. i do vaguely remember reading threads about cases where impeller magnets have demagnetized over time.
the 'little plastic bump' is there to give the impeller some freedom in spinning so it can pick a direction without the resistance from the impeller blades easier and get started with fewer convulsions. the chatter is when it can't decide what direction to go in, so it slaps back and forth.
so, how would lowering the voltage slow the pump down? i'm not saying it doesn't, but i don't think that would work with an AC induction motor. sure, lowering the voltage would weaken the electromagnets which would then exert less force on the impeller. but the impeller has to advance from position to position to match the frequency of the changing fields. with a weaker field, it might not make it to the next position in time and it might actually change direction. seems to me, it'd have to be all in the frequency.
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