High-pitched whistle/hum vibration sound

[Squarebear]

I can hear it now. 3:40am. Sketty. It's coming from east of here, I reckon. It lasts a couple of seconds. Then it's quiet for a moment, then repeats. It resembles that noise made when you wet your finger and rub it around the rim of a glass.

Anyone else hear it?

 

[MrSwerve]

Tinnitus?

 

[Muteswan]

I’m sure it’s the fog warning. I’m closer to the sea than you and it’s been going for hours.
It sounds louder at night as there is less noise going on.
I can still hear it as I type.

 

[Squarebear]

Aye, cheers. Someone else on Fakebook also said exactly that.