You know my 235 in my '46 2 ton was a noisy chattery critter like that too at first.
Like you, i'd not ever heard a "right running" one to know if the chatter was normal or not.. it had a kind of diesel motor sound, but otherwise ran great.
What I discovered was that these motors are supposed to have pretty heavy oil in them.. and i'd put 5W-30 in it with the first oil change.
Putting heavier oil in it quieted it down quite a bit.. it still chatters a bit, but not nearly as much as it did with the thin oil in it.
