Imagine, as your pipes get older and the corrosion and erosion takes it's toll on them. The least bit of trash in the lines could hang up anywhere and become a reed style device. The changing pressures combined with the rate of flow, the changing temps of the water could all be factors in the pitch, the volume and the intermittence of the hum. Trash in the lines could be anything from scales from steel pipe, excess glue from a plastic joint, teflon tape partially bridging at any threaded joint. Sounds telegraph easily through water pipe. I'd be interested in back washing my system or at least removing the supply lines at the stops and flushing my system one fixture at a time into a 5 gallon bucket. You might be surprised how much trash you get out of those old lines.