What was found
EXPERIENCED VERY HIGH FREQUENCY VIBRATION AND WHINING SOUND AFTER TAKE OFF FROM THE BASE ENROUTE FOR AN INTERFACILITY TRANSFER. TOOK IMMEDIATE ACTION AND LANDED BACK AT THE AIRPORT SAFELY. WHINING AND VIBRATION WAS NOT ACCOMPANIED BY ANY CAUTION LIGHTS, OR FLUCTUATION IN ANY OIL TEMPERATURES OR PRESSURES. WE PERFORMED INSPECTIONS OF THE ENTIRE AIRCRAFT AND FLIGHT CONTROLS. WE PERFORMED OPERATIONAL CHECKS OF ALL THE HYDRAULICS. WE PERFORMED TRACK AND BALANCE ON THE GROUND AND IN FLIGHT. WE PERFORMED THREE DIFFERENT OPERATIONAL CHECK FLIGHTS AND COULD NOT DUPLICATE THE SAID DISCREPANCY.