A rod knock is caused by excessive oil clearance, either from a badly worn bearing, an out-of-round crankshaft, or both. The oil film between the crankshaft and the bearing can't cushion the rod's tendency to hammer up and down as the crankshaft turns. That problem can turn serious in a hurry, with a ruined crankshaft, a locked-up engine, or even a broken connection rod that knocks a hole in the block, so don't continue to run it much until you discover the cause of the noise. It's not going to get better by itself.
Jerry
