"banging Pipes" During Spin

I'm no expert but I've just got rid of a Hotpoint WMF 740 which had exactly the same problem. The noise was during the spin cycle and was a long way from the machine. The pipes had no detectable movement in them nearby so I ruled out the possibility of the machine striking the pipework itself.

My new machine has brought silence back to my house.

My theory for why this was a problem during the spin is that the inlet valve has probably got a bit dirty (hard water where I am) or is wearing out. I assume that the valve stays closed due to the action of a spring which is now struggling to keep the valve closed tightly when the machine is being shaken at 1400rpm. I don't think it needs to open much to cause a pressure wave in the pipe.

I'm no engineer though, so I'd be interested in a more authorative reply from an expert.

My guess is that some vibration from the machine on spin will inevitably be passed on through the pipes even if they don't sway about. If you hold the pipes whilst the machine is on spin I'm pretty sure you would feel vibration in them. This will also be passed on to the plumbing pipework and if loose somewhere can cause them to vibrate or knock. With a large pipe network it can resonate, especially if the washing machine creates a certain frequency, which a new or different one doesn't.

Sometimes a constant knocking noise can be set off by the cold water valve shutting off in high water pressure situations with loose pipes although that should be noticed before spin started so it's something I mention just as an extra cause of plumbing knocking for anyone researching the problem.

Good point, I should have said that we still get the same knock from the pipes when the valve shuts after a fill - even when the drum is still and even with the new machine. So I still think water hammer is the right explanation for the noise in my case. The only mystery to me was how it could have started happening during the washing machine's spin cycle (it wasn't always like this). I know I should get that pipe secured but it is in a hard (and costly) place to reach.

Pulling the machine out and operating it so you can clearly see that it isn't touching any pipework should be enough to rule out vibration through the pipes I would have thought, assuming there is plenty of flex left in your flexible rubber hose!

As long as the hoses are connected to the washing machine and the plumbing, vibration will be transferred even if it's very small fast vibration that doesn't seem to move the hoses much. If your new one doesn't do it, it may be spinning at a different frequency and not creating the right vibrations to vibrate the plumbing pipes.