The present invention relates to a device for automatic injection of a product into an injection site, the device comprising: a container having a displaceable piston; a tubular housing having at least a flexible part which, in a rest state, defines a first opening; a piston rod; and, an intermediate part fixed to the piston rod which defines a diameter larger than the first opening. With sufficient distal displacement of the piston rod, the intermediate part causes the at least a flexible part to deflect outwardly with further distal displacement of the piston rod causing the intermediate part to wholly pass through the first opening with the at least a flexible part returning towards the at rest state, engagement of the at least a flexible part with a surrounding element under force of movement towards the at rest state generating a sound.