I would look for a slow leak somewhere, follow the noise. I was awakened
one night years ago by a faint buzzing kind of sound. Actually it was so
faint, I'm not sure it was what woke me up, but once up I noticed it.
I tried to follow it, thought it was inside the house, then checked outside,
finally went down to the basement. The water heater had started to leak
and the slow release of water was what was causing the pipes to vibrate.
Fortunately I caught it while it was just a large puddle, not a flood.
So, I'd look for anything that could be drawing water, eg leak, partial
open outside faucet, etc.