Shaking Bed

Last night as I was drifting off to sleep my bed started vibrating. (And before you ask, my hero is away and I was sleeping alone).
This isn't the first time my bed has shaken (really you want to go there?) but I have been noticing it happening a lot lately. I don't recall it happening if there is anybody else in bed with me (oh, grow up!). Nor can I remember it occuring with any light on (either the room light, bedside lamp or light coming in from the hallway or ensuite). While last night it hapened as I was falling asleep in the past it has happened when I've been awake and once or twice I have been woken up by it.
It's a difficult thing to describe. If you've ever experienced a mild earth tremor it's sort of like that but not quite. It is definitely the bed moving and not me. It feels like a gentle shaking that increases in intensity but never really gets too vigorous.
I'd love to know what could be causing it or failing that if anybody else has experienced anything similar.