Re: American fan in the UK.
that sounds like what I call 'drumming', one of my desk fans does that (and it's a modern one). I suspect end-play in the main shaft bearings casuing the rotor to float about a bit. It sounds like your casing is resonating and adding to it.
Maybe a strip down and lube would help, or try reducing the mains voltage with a variac to see if it stops the floating
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Kevin
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