Classic/Antique Car Repair: clutch noise?? 51 chevy, throwout bearing, snap ring

Question
QUESTION: i am having a problem with a "growling" noise. tranny was rebuilt and shifts smoothly. rear end has fluid as does tranny. noise whines/growls when you let off clutch upon acceleration and continues while driving. it goes away when you push in clutch and at idle/neutral. throwout bearing is also new and so is pilot bearing. i can't figure it out. can any one help???

ANSWER: Is the noise there when running the engine with the transmission in neutral and your foot off the clutch? Let me know.
Brad

---------- FOLLOW-UP ----------

QUESTION: no just when your driving down the road. or if i have the rear end up on stands. seems like only when there is a load on transmission. sorry engine and tranny is a 51 chevy 216/3 speed manual.

Answer
As I see it the problem could be the following:
The rear bearing area including the snap ring popped off, the U joint spacer missing, or the U joint of ball adjustment.
Now because the noise will happen with the engine running in gear with the vehicle on stands, I would check it with a mechanics stethoscope. It will pin point the noise.
Brad