Pump for applying an increased or reduced pressure to a container. The pump is provided with a plate-like part which reacts to the pressure difference between the container and the environment. This part can preferably adopt two stable positions, and a change is signalled. This change can be brought about, for example, by the sound generated as a result of the plate-like part turning over. The pitch and the like of the sound can be influenced by arranging the plate-like part in a hollow piston and providing the latter with openings leading to the environment.