I know what you mean. It's quite relaxing in a funny way.
I've noticed it for years. Some weeks it's around on and off and then I don't get it for ages. Weird!
I can't really connect it to anything except once or tiwce it has preceeded a headache.
It's reassuring that somebody might have the same thing. Maybe you should type it into a search engine and see what comes up!
Good luck