In an internal combustion engine the power generating parts mainly responsible for body resonance and sound vibration such as the crankshaft (3), cylinder block and cylinder head (35) are combined in a power unit which is supported in the crankcase (4) on elastic vibration-absorbing mountings (2) with the sidewalls of the crankcase (4) extending above the crankshaft axis (3).By this arrangement the crankcase (4) is free from direct vibrations leaving other non-power transmitting engine parts mounted above the crankcase (4) which require means to suppress radiated noise.In one embodiment the induction manifold (7) and mounting parts (13, 14) thereof together with the rocker cover (20) are insulated from the cylinder head (35) by vibration-absorbing elements (8,15,16,21,26) and are so shaped and arranged that they provide a screen for noise radiated from the cylinder head (35). Thus, essential parts of the engine have the secondary function of reducing the level of noise emitted by the engine.