A little more about loose impellers:
When I was testing a little Hydor, I found that the max head was nowhere near what it should be (1.35m). In fact, it was fluctuating wildly about the 1m mark, and the rattling seemed to start whenever the head would reach a certain value. So it seemed like the pump wanted to, but the rattling was somehow stopping it.
Anyway, I fixed the loose impeller with a tiny blob of Blu-tack, and miraculously the head then maxed out to 1.33-1.35m and the pump was completely rattle free. Luckily, it also seemed to make no difference to starting performance.
I like your explanation about play allowing the rotor to spin up before it has to do any work, I also wonder if maybe these aquarium pumps are intentionnally designed to rattle, to alert you to a blocked filter or something like that? (aquariums have filters, right?)
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