Piping describes a noise made by virgin and mated queen bees during certain times of the virgin queens’ development. Fully developed virgin queens communicate through vibratory signals: “quacking” from virgin queens in their queen cells and “tooting” from queens free in the colony, collectively known as piping. A virgin queen may frequently pipe before she emerges from her cell and for a brief time afterwards. Mated queens may briefly pipe after being released in a hive.
Piping is most common when there is more than one queen in a hive. It is postulated that the piping is a form of battle cry announcing to competing queens and the workers their willingness to fight. It may also be a signal to the worker bees which queen is the most worthwhile to support.
Here is a wonderful video done by HCBA member Phil Serafinas: