Queens will pipe inside the swarm cell.It sounds as though your bees may be preparing to swarm. Action may be needed soon.Perhaps your hive has a virgin queen in with a mother queen?
Taken from here:http://ag.udel.edu/enwc/faculty/dmcaron/Apiology/queenswarms.htm
There are many sounds we can hear in a bee hive, do bees hear them too? Bees lack ears or other sound-capturing structures but they do hear! Bees communicate with sounds both in queen rearing and in dance language communication.
In swarming and supersedure, "piping" is a high-pitched sound produced by queen muscle contractions without unfolding of the wings. The thorax vibrates faster with wings folded than when unfolded so the sound is not the usual bee "buzz" but a high pitched "piping" sound.
The queen, as she pipes, presses her thorax against the beeswax comb. Adult queens pipe on or close to the queen cell of developing queens.Worker bees pick up the sound, probably via vibrations, and may be observed to stop or freeze movements in the vicinity of queen piping. The adult queen pipes for a two-second pulse followed by a series of quarter-second toots. If there are virgin queens within queen cells, they respond with a series of ten short pulses.
Piping is more frequently heard in swarming than in supersedure behavior and is more commonly heard after the primary swarm leaves. We do not know what precise role it plays but it is believed piping may help time swarm departure, particularly for afterswarms. Also, it may help the virgin queen locate her potential rivals so she can eliminate them.
The ordinary buzzing sound made by bees when flying may or may not be perceived by bees. If we hold a worker bee in our fingers she too will make a high pitched sound somewhat similar to queen piping. Worker bees will also emit this sound in the hive-perhaps as a warning or alarm sound.
Sound production is vital in dance language communication behavior. Worker bees must precisely time the length of waggling since it encodes the distance to food source portion of the message. The unique noise of the "breaking" dance, signaling swarm departure, may also be a sound the bees can hear.
The perception of substrate "noise" may be via touch receptors rather than airborne sound wave perception. Beekeepers know that jolts and vibrations to the hive serve to alert a bee colony and may result in more stings during colony inspections. The ordinary background hum of bees may likewise be a touch stimulus transmitted through the beeswax comb.