Update: still humming almost a week later, but the water board finally had
an operative free.

In . net,
Christian McArdle wrote:
It is quite common for some piece of electrical equipment to come in
contact with the water pipe and resonate it. You are likely to get
either 50Hz, 100Hz or harmonics thereof. It can be intermittent as
the equipment touches or otherwise as things move or expand with
temperature etc.

Apparently it has affected quite a wide area; an estate built a few years
ago with all-plastic plumbing is also vibrating.

The water board sent a guy out. He scratched his head and called in a guy
from the electricity board. They scratched their heads in stereo. It seems
my house is the worst affected. They didn't stay here long.

They've had a close look at lamp-posts in the area, and were last seen
eyeing a nearby substation with suspicion. Daresay they've gone home for
the night now.

*shrugs*

Gill