From time to time I get a humming noise in my pipes that lasts for a few seconds and then fades away. So far I have not been able to locate a source location or specific event (ie running hot water) that triggers these events. The number of these events is slowly increasing.

I'd appreciate any advice on diagnosing the problem and/or solution.

Humming is like a slight vibration and can be caused by a vibrating washer in a valve. Try every fixture in the home and track down location, then put your hand on the supply pipe or valve so you can feel the hum.