THE HELICOPTER WAS HOVERING ABOVE TREES AT ABOUT 100 FT AGL WHEN THE TAIL ROTOR CHIP DETECTOR LGT FLICKERED SVRL TIMES, THEN ILLUMINATED STEADILY. THIS WAS FOLLOWED BY A LOUD GRINDING NOISE FROM THE TAIL AREA & THE ACFT YAWED TO THE RIGHT. THE ACFT ENTERED AN AUTOROTATION AFTER MAKING 3 TURNS TO THE RIGHT. IT DESCENDED INTO THE TREES AT WHICH TIME THE MAIN ROTOR BLADES WERE BROKEN OFF. AN EXAM OF THE INTERMEDIATE TAIL ROTOR GEAR BOX DISCLOSED THAT 6 TEETH IN THE INPUT BEVEL PINION GEAR, PN S1635-64114, HAD FAILED IN THE ROOT AREA. A METALLURGICAL EXAM DETERMINED THAT THE FAILURE ZONE HAD FATIGUE INDICATIONS. CRACKS WERE FOUND IN OTHER AREAS OF THE GEAR BOX. SIKORSKY ENGINEERS DETERMINED THAT THE GEAR BOX HAD BEEN SUBJECTED TO HEAVY LOADS. THE GEAR BOX HAD BEEN LAST OPENED IN 1968. ONE GEAR WAS NOT A SIKORSKY PRODUCT.