What was found
LOW ROTOR RPM LT AND AUDIO INTERMITTENTLY ACTIVE WHILE HOVER TAXIING, ALL OTHER INDICATIONS WERE NORMAL. A TESTFLIGHT EVAL FOUND UPON LANDING ON THE PAD AND REDUCING COLLECTIVE, THE ROTOR RPM AUDIO AND LIGHT ILLUMINATED WITH ROTOR RPM AT 96.8%. WHEN INCREASING COLLECTIVE THE ROTOR RPM LIGHT STAYED ON AND THE HIGH ROTOR RPM AUDIO ACTIVATED AT 99.4% RPM. WHEN DOING AN OPERATIONAL CHECK OF THE RPM WARNING SYSTEM BEFORE FLIGHT IT CORRECTLY ACTIVATED AT 95.1% RPM. ACFT WAS TESTED WITH A SIGNAL GENERATOR AND WORKED CORRECTLY INDICATING THE SENSOR IS GIVING ERRONEOUS FREQUENCIES CAUSING THE WARNING SYSTEM TO FUNCTION INCORRECTLY. REMOVED AND REPLACED NR SENSOR IAW AMM 62-41-00, 4-1. MOC SUCCESSFULLY COMPLETED.