Well here's an oddity - which I forgot to report: when opened, I can hear a distinct, low thrumming from inside one of the hives!
The sound is similar-ish to the sound of a dove, but faster and lower.
Could it be a mouse? There is no sign of mice, and the anti-mouse bars were closed all winter (and still are).
Could it be the queen? Various bee books refer to the queen making cicada-like sounds. Do queens have timbals? Can they stridulate?
Competitive Scrabble, anyone?
Pic from wikipedia, more about Scrabble at http://www.scrabble.com/