THE HELICOPTER (HEL) WAS BEING USED TO TAKE A RADIO TO A COUNTY EMERG OPNS CENTER TO TEST ITS RECEPTION AT 500' AGL. LOADED WITH THE RADIO, 27 GAL OF FUEL & A PAX, IT WAS WITHIN ITS WT & BALANCE LMTS. AS THE HEL WAS APCHG THE 500' LCTN &WAS BEING SLOWED IN LIGHT WIND (EST FM 160 DEG AT 5 KTS), THE PLT FELT A MODERATE VIBRATION. HE INTERPRETED THIS AS A TAIL ROTOR PROBLEM & REACTED BY ADJUSTING THE THROTTLE & OPERG THE PEDALS. HE STATED THAT THIS SEEMED ONLY TO OSCILLATE THE TAIL & CAUSE A LOSS OF LIFT. ALSO, HE NOTED THE VIBRATION SLOWED IN FREQ & BCM MORE VIOLENT. HE ATMTD TO INCREASE ENG RPM & RAISE THE COLLECTIVE TO GAIN ALT, BUT THE HEL BGN SPIRALING. IT THEN CRASHED ON WOODED TERRAIN & BGN BURNING, BUT THE OCCUPANTS EGRESSED. NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND THAT WOULD HAVE RESULTED IN THE ACDNT, THOUGH SOME BINDING WAS NOTED IN THE PITCH CHANGE MECHANISM. SVRL WITNESSES INDCD THAT BEFORE THE ACDNT, THE HEL APCHD THE AREAFM THE NE & BGN SLOWING, AS IF THE PLT INTENDED TO HOVER. IT THEN BGN ROTATING TO THE RGT & SUBSEQUENTLY CRASHED.