I had refurbished my old distributor on my 235 according to shop manual and then reinstalled in engine, When I fired up the engine for the first time I got the same result loud popping or tapping on the distributor side. My oil pressure was some what low but close to normal. I put my hand on the distributor and could feel a definite vibration. I shut down the engine and pulled the distributor, backed off screw on the vacume clamp and reinstalled insuring the distributer seated all the way to the bottom into the oil pump tightened the vacume clamp and dist bolt. Presto problem solved. My distributor was not fully seated into the oil pump and poping up and out as it turned, the result was the loud noise you discribe. I do not know if this is your problem but I would suspect that it is. You can use a long scew driver to line up the oil pump connection with your distributor shaft. good luck.
