The present disclosure relates to an automatic feedback mechanism (10) for a medicament delivery device that notifies a user of a start of expulsion of medicament from a medicament container. The automatic feedback mechanism comprises a plunger rod (11), a tubular extension member having a radial space defined by an inner perimeter and an inner distal surface, wherein the tubular extension member is arranged to receive the plunger rod (11) in the radial space, and wherein the tubular extension member has flexible tongues configured to releasably lock the plunger rod (11) in a pre-tensioned state, a tubular operation member arranged to receive the tubular extension member, which tubular operation member is movable relative to the tubular extension member, wherein the tubular operation member in a first position is arranged to press the flexible tongues radially inwards to lock the plunger rod (11) in the pre-tensioned state, a first resilient member (15) arranged to be received inside the plunger rod (11) to exert a force in the proximal direction, and a signal generating member (21) arranged to partially surround the plunger rod (11) and having a distal transversal end portion (21a) which in a first position is at a distance from the inner distal surface of the tubular extension member when the plunger rod (11) is in the pre-tensioned state, wherein in an assembled state, movement of the tubular operation member causes the flexible tongues to expand radially releasing the plunger rod (11) to travel proximally and allowing the first resilient member (15) to move the signal generating member (21) distally the distance such that the distal transversal end portion (21a) hits the inner distal surface generating an audible signal.
