Many thanks Zipper.
Actually I'd neglected to mention that this banging and leaping effect is accompanied by a very rapid spin of the drum, which would lend credence to your line of thinking above.
As I have no expertise to speak of, and as it's a very basic machine anyway, I think I'll...
When I say 'leaps' I'm not referring to the problem that some machines have of hovering about, and when I say 'bangs' I'm not talking about persistent banging during the spin cycle. I mean when I put it on to wash, the machine fills up with water and then does nothing for about 15 seconds except...