Whirling/Squealing from 727 -- help!

Here recently my Dart has started to make a rather high pitched whirling/winning noise. At first I suspected the rear brakes but they have all new hardware, and don't show any unusual wear. The brakes never get hot and there are no out-of-place shiny spots on the drums. While cruising with some buddies last night it started to do it again. Sometimes it comes and goes but seems consistent when hot. It started to do it when I was slowing down to pick somebody up so I had my buddy hold is foot on the brake but let the car move slowly. I jumped out and crawled along beside it and now I'm thinking that It's coming from the transmission, possibly the tailshaft area. I've never had problems with the transmission since it was rebuilt. It is full of fluid and I went ahead and greased my exhaust bearings to make sure it wasn't them.

What could it be?