As some of you might know, I always work towards "original when street restoring my bikes. Hence, my goal is full Gran Sport and working properly...
Randy, your post provided the nudge I needed to update my Gran Sport pulley thread. You might want to look at my most recent post on that thread and check to see if your pulley slots are aligned with the nubs on the cage plates.
Also, these derailleurs are fifty plus years old. Some of them have lived a hard life. One of mine was giving me the sort of trouble you describe because the pulley cage was out of alignment. (two pulleys were not running in exactly the same plane) A rather crude adjustment to the lower pulley cage with a 10" crescent wrench quieted it right down.
Brent
Now that is something that I did not look into. Great advice and easy enough to check. My intention is to rebuild the Gran Sport and, hopefully (definitely), use it. I have, honestly, never run into this problem before and the noise, telegraphed from the rd to the crank set, proved my nemesis - so far. What most amazed me, and still does, is the way the noise telegraphed from the rear derailleur to the crank area.