The sub-transmission is at the rear of the engine which is just a couple of fairly large gears for L,H & R I guess the noise could be a bearing but it sounds to me like the transmission .. The oil pressure for that transmission calls for 22lb at idle (1400 RPM) & 116lb at 5000 RPM .. But those numbers are low ,we check the oil pressure on every TRX500FA/FGA that comes in for service .. The oil pressure is usually 40-60 LB at idle & 160-180 LB at 5000 .. If that transmission doesn't receive high oil pressure it will make a ratcheting sound at take off ,next it will make a sandy kind of grinding sound all the time and then it will need to be replaced ..
If the oil leak isn't fixed you will soon need a new transmission .. The leak should be fixed and the oil pressure tested ASAP ..
If the oil leak isn't fixed you will soon need a new transmission .. The leak should be fixed and the oil pressure tested ASAP ..