I had to stop using koralias after I got a wave-maker for the very reason you're describing.
I'm not sure if the model you have has the same problem, but the Koralia 1, 2, 3, etc. series have magnets that aren't snugly secured in their housing, so that when they start up, the impeller assembly gets pulled up the pin and away from the housing until it creates enough back-pressure from the spinning impeller to push the magnet/impeller assembly back down and centred in place. As the impeller assembly gets sucked upwards in that first second, it rattles all over the place. Sometimes the noise isn't very loud, and sometimes it sounds like the impeller is going to break, but with the models I had, it made a noise every single time they turned on (which, as you can imagine, was infuriating with a wave-maker).
Someone on RC figured out that if you thread a small piece of airline tubing onto the pin the impeller spins around, and put it between the top of the impeller and plastic housing of the pump, it stops that rattling from happening. Unfortunately, it also seems to cause the impeller to spin backwards about half the time.
Anyway, I'm not sure if that's the same problem with that model of pump. Because of that issue with my Koralia 2s, I now have a whole tupperware container of koralia pumps collecting dust in a closet...
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