Transmission Noise

Tell us more about it. When exactly does it do it? At idle, when you give it some gas, or all the time? Mechanical noises can travel around through metal parts making it difficult to tell exactly what part is the culprit. Even though it sounds like it's coming from the back of the engine remove the belts and start the engine for a minute to verify it isn't any of the accessories. Is there any possibility you left a vacuum line off the back of the carb when you put it back on? A vacuum leak can whistle pretty loud sometimes. From your description that may not be it but it's worth a check.

If the car has been setting for quite a while it's possible the torque converter drained back (common malady with torqueflite's) and got air in the transmission hydraulic system. Air in the system can make an awful squealing noise. Torqueflite's don't fill the converter in park so you may try putting it in neutral and running it for a while to see if the noise goes away.