Here's one possible source for the noise- - - -the camshaft has some flat spots between lobes for connecting rod clearance. Yes, it's that close down there at times. If the cam isn't turning due to a stripped timing gear, it's possible the rods are hitting the cam as the crankshaft spins. Is the 235 actually an "interference" engine where pistons and valves can come into contact with each other? I sort of doubt it unless the head has been milled and/or a high lift cam has been installed.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!