A ten minute flight in the local area had been completed without incident. After DEPARTURE for a second flight the helicopter was climbed to 150 feet agl to allow the passengers to view the campsite and a herd of animals. The pilot and passengers then heard a loud bang which was followed by a severe airframe vibration. The noise and vibration continued and the pilot elected to carry out an autorotational descent and land in a small clearing. The clearing was overshot and the helicopter struck several trees. Investigation indicates that the engine cooling fan drive belt, which rotates at high rpm, failed. The flailing belt then tore a section of the canvas cooling fan shroud. Vibration was produced when the broken belt and section of shroud were ingested into the cooling fan.
