Cooling

Not an evening for shorts - my mistake.

At around 3am our water pipes started making odd noises. I checked all over the house, turned taps on/off, flushed the loo, switched off power - the noise continued. So, I turned off the water at the mains and it stopped.
This morning I turned the water on again and the noise returned. I put the washing machine on and the noise stopped and hasn't returned.
Can anyone explain?