I had a very similar un-explained noise shortly after rebuilding my th350 too. A very high pitched squeel at all times. It got worse as the engine temp went up. I stopped by a tranny shop and they said its most likely the converter. They surmised that it was put together with too much preload on the torrington bearing inside. I returned the converter under warantee and the new one is fine now, no squeel since changing it out.