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I don't think that rattling is absolutely indicative of a problem. I wondered how these buggers worked, so I took one apart (and I put it back together again, and it still worked...). It's pretty ingenious - a Zener diode (probably at 12 volts or so) powered through a resistor operates a relay coil. If voltage is under 12 volts, the Zener shorts the whole thing to ground and nothing gets through to power the coil. If the voltage goes significantly higher than that, the coil is energized enough to pull the main 12V (usually going to high-value electronics) to ground and blow out the fuse.
But the rattling in my case was a piece of phenolic insulation thrown between the bottom of the printed circuit board and the metal case of the OVP unit. Because rattling generally is associated with "bad", I'll bet later units are built not to rattle.
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