In message , Brian Gaff
writes
Well there is no CU I have storage heaters here. I turned off the fridge and
there really is nothing else. Its only on for a few seconds, probably less
than a minute, then it might be off for two minutes, then on for one again.
its almost as if its topping something up.

I too have occasion to leave my bed in search of non-directional noise.

The Bristol report sounds plausible. He didn't mention that your brain
must already be ignoring noises made by your own body; heartbeat, blood
flow, digestion etc.

Like you I have recovered from a spell of tinnitus. Mine triggered by
muzzle blast from an over close 12 bo-(

I think if you are confident the noise is from next door you could
approach your neighbour on the basis that something abnormal is
happening which might be costing her money rather than it interfering
with your sleep.

Lots of excellent suggestions but none that match your observed timing
if operating normally. On the water side, I suppose a bad leak to the
pan from a toilet cistern might trigger some cyclic make up noises. The
solenoid valve on our dishwashers causes an audible thump from water
hammer which can be a bit unnerving.

One item I suspect of 100Hz hum below audible level is the cold cathode
tube clock display sat on my bedside chest of drawers.

--
Tim Lamb