What happened to your old distributor?

Tranny isn't likely at all, if it were, it would be the clutch, and not the gearbox portion of it. see if it makes that noise both with the clutch pressed and depressed. If it makes the noise doing one, but not the other, it's your throwout bearing in the clutch. If the car makes this noise when it's sitting still, with the clutch pedal in either position, then it can't be any portion of your transmission.

Why do you suspect the tranny? Is it doing something weird, too?