A signalling device (10) adapted to make a percussive sound, for example, under water. The signalling device (10) may be attached to a pressurised fluid source, such as a diver's compressed air supply, the device (10) including: a housing (12) having a passageway (22), at least in part, for communication with the fluid source; a valve member (34) located in the passageway (22) and movable between a closed position in which it substantially blocks the passageway (22), and an open position in which fluid may flow through the passageway (22); biasing means (42), such as a spring, biasing the valve member (34) towards the closed position, wherein the valve member (34) reciprocates between the open and closed positions when a pressure difference above a set value exists on either side of the valve member (34) to produce a percussive sound.