Ive rebuilt several shovels,your crank pin could have a slight flat spot in a small section anywhere around the pin ,so you must set a good dial indicator on the frame to the top of each rod checking up and down rotating very little at a time and rechecking you cant just check it in one little spot. note if there was a pinging noise at mid rpms that sounded like it was at the middle of the jugs theres a good chance there is a bad spot in the crank.the noise travels to that area from the cranks bad spot ,hope this give you some direction.
mark
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