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@Dammit
Also, some thoughts- I don't think it's the axle moving laterally, it's shimmed to ~0.3mm of float which is controlled by a highly compressed wave washer, and the noise and behaviour hasn't changed with different numbers of shims.
There is slack in the drivetrain- as you push down on the pedal from a standstill you can feel this slack being taken up before you put drive through the chain, accompanied by "cra-crack!", if you back off at once and freewheel you get a "ping!" As whatever it is springs back into the "slack" condition.
So there is something which moves under load (300 watts up I would say), and returns to its previous state at high speed when load is removed.