Older Emperors can be a pita in this regard. It is not always due to bent shaft or worn rubber. There seems to be an issue with two other parts- the flow control mechanish and the intake tube end inside the filter. You may be able to quiet it down by messing with the flow control level and pushing down/back on the top of the intake tube. You are basically trying to find a happy position between where the end of the tube meets the flow control piece.
I have also found with mine (I have it for 7+ years now) getting noisy after a thorough cleaning where I pull the whole filter and bleach soak it. It seems to rattle a bit until some new gunk builds up in the impeller area- kind of a natural, slimey lubricant.
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