Another possibility is a deteriorated harmonic balancer. That's a 2-piece unit with rubber dampers built in. If they get worn to the point of allowing metal-to-metal contact, a hammering noise can be the result, although it would probably be a lot higher frequency. A Buick 3800 V-6 was prone to that type of failure on a pretty regular basis. I've never seen that on a stovebolt engine, but there's a first time for everything!
Jerry
