Abstract: A silent-running motor power unit with an internal combustion motor (3) enclosed in a sound absorption chamber formed by a casing, having at least two walls (24, 22a, 23) arranged one (24) inside the other (22a, 23). The walls form part of a labyrinthine channel (6, 1) lined with sound absorption material. To make access and service to the motor (3) easy said walls (22a, 23, 24) include inner (24) and outer (22a, 23) door sections hinged at one of their ends whereat an inner section (24) may be swung outwards through the opening of an outer section (22a, 23). This is achieved by making the sections as a smaller part of a channel wall and as a light plane wall section instead of previously used heavy channel volume type sections.