Squealing or siren-like sounds?

A727 by TCI, 340, '69 Dart. Very low mileage. I have a squeal, or almost siren-like sound when the engine is running. Have completely ruled out the p.s. pump, water pump, and alternator. Can't pinpoint the source of the noise, but it seems loudest before everything warms up. Comes and goes, and changes pitch/tone, or goes away when you put the car in gear, out take it out of gear. Someone told me it's a bad pump in the transmission. Someone else said it's a bad torque converter. Car runs and drives just fine, and I think the noise goes away when the car is moving. Loud exhaust may be drowning it out though. Any thoughts?