THE TRAFFIC WATCH HELICOPTER WAS CRUISING NORTHBOUND ALONG THE INTERSTATE AT 450 FT AGL & 85 KTS AIRSPEED. A GRINDING NOISE WAS HEARD & THE TAIL ROTOR SEPARATED FROM THE ACFT. THE ACFT BEGAN AN UNCOMMANDED RIGHT TURN WHICH WAS ARRESTED IN ABOUT 1/4 TURN. THE PLT ATTEMPTED TO AUTOROTATE INTO AN OPEN FIELD JUST EAST OF THE HWY, BUT STRUCK A 25 FT TREE AT THE EDGE & CRASHED. AN EXAM REVEALED THE TAIL ROTOR GEAR BOX HOUSING, P/N 369A5400-607, HAD FAILED. A PORTION OF THE HOUSING, DRIVEN GEAR, TAIL ROTOR BLADES & RELATED COMPONENTS SEPARATED FROM THE ACFT IN FLT. THE MISSING PARTS SANK IN A LAKE ABOUT 1/2 MI FROM THE CRASH SITE & WERE NOT RECOVERED. THE REMAINING SECTION OF THE BOX WAS EXAMINED BY NTSB LAB & FAILURE WAS DUE TO OVERLOAD WITH NO EVIDENCE OF FATIGUE OR CORROSION. A WITNESS SAW 2 OBJECTS FALL FROM THE HELICOPTER. ONE OBJECT WAS WHITE AND LOOKED LIKE A BIRD, PROBABLY A SEAGULL. THE OTHER OBJECT LOOKED LIKE A STICK OR A 2X4 PIECE OF WOOD.