Don I had a 302 crank years ago on a 289 block(stamped 302) where the valves were hitting the piston. I am sure by reading what you have said this isn't the case for you since you didn't hear anything the first go round. I'm also wondering about the screw in plugs, since that's the only difference between before and after, unless something got bent? This will be a good diagnostic thread to follow! Hope it's simple and inexpensive.