[AndyH]

Well...the best way to get to the bottom of it is to do a little investigation.
If your wife also hears it you can probably rule out tinnitus.

Do you both have to put up with loud noises during the day?
Do you both have very sensitive hearing?

It could be residual ringing from sustained noise during the day or an electrical device.

The electrical idea can be ruled out by simply throwing the mains switch so that all devices are off and then seeing if the noise goes away.

I remember when I used to live in a city for a short while when I was younger I had a real pig of a time with my ears ringing. My dad fixed it by putting a small fountain in my room. It sounded like rain hitting the window which used to work a treat for me out in the countryside, it brought back all the memories and I slept well...although I did get bothered there by sheep bleeting all the blasted time lol.