Was the engine rebuilt? I recall having a similar sounding problem some years ago. It took awhile to figure out that the rebuilder, a previous owner, had used the incorrect rocker arms and had even installed those incorrectly. Being a neophyte to the hobby, I learned the hard way, parts between different production years of the "same" engine, in my case also a 216, are not necessarily interchangeable. This '39 version, as I recall, had rockers from an early '50's 216. Oddly, the engine seemed to run just fine and without my having heard the valve clatter previously until one morning on a road trip. A seasoned mechanic buddy of mine helped solve the mystery and sort the mess out.
