I wouldn't say throw-out bearing if it is only under load. A friend in town has a bad throw-out bearing making serious noise at idle that goes away as soon as the clutch is depressed. My guess is a pulley/bearing issue since you only hear it under load. Try the old "stethoscope" method with a long screwdriver or piece of wood. My money is on an idler pulley or the water pump. Best of luck with it.