68 Dart starter issues - any ideas

Well you just narrowed it down a lot.You proved the clatter is right at the convertor to pinion gear interface. Barring a low battery, allowing the pinion to drop in/out, and since you are on the second starter, Ima guessing there is something wrong with the RG.It sounds like it is too big on the OD, or wrong tooth count.SBM cars are 130tooth IIRC.