I can't believe this!!! I had exactly the same yesterday and almost posted this:-
The sole of my left foot is "trembling" ever so slightly, it feels as it would if it was resting on something that was vibrating because of loud noise, and yet there is nothing providing any vibrations at all; the feeling is still there when my foot is off the ground too. Also there is no visible sign that anything is happening e.g. no obvious muscle activity.
It's not in the least bit painful but it is continuously noticeable having been present for at least an hour so far, and it is only the left foot. Also I have had sudden, painful cramps in that same foot many times over the past year, but that might just be coincidental.
I'm not worried about it, for all I know this happens to everybody and I'm just aware of it for the first time, but I was wondering if anyone might have an idea what might be causing it