Humming Pipes

Asked to help out to stop pipes humming at random times, so difficult to locate problem as it didn't happen when I first visited.
TH-cam videos suggested valves were not fully open, so I opened all the valves with no change. I also checked the boiler and also the toilet was not causing the problem, but both OK and not the issue.
Luckily it happened while I was there on my second visit and traced the noise to the water meter outside.