I'd concur with the two previous posts. If the T/R and/ or gearbox are not producing any "secondary" indications such as vibrations, metal or excessive heat, I'd look elsewhere, and as "formerlongbox" suggests, eyeball the engine. Another suggestion; years back I had an issue with an oil cooler blower or "squirrel cage". The impeller had somehow received a slight deformation to a couple of "vanes", which set up what was described as a shrill whistle at full RPM. Hard to pinpoint as it couldn't be heard from the front or on the ground close up as the engine and rotor noise masked it pretty well, but it could be heard from a distance aft and away from the helicopter. Good luck.