If you blew the head gasket while it overheated and coolant got in the oil there is a good chance you spun a bearing, also when things get hot they expand and the bearing could have definitely hit the crank and all it needs to do is touch the crank for a micro second and it will spin. Look up rod knock sounds and compare. I would check the oil and if it doesn't look right there is coolant in it meaning the head gasket when and coolant has no anti friction properties so when it went through the system it could have went to a rod and the rod hit the crank and the bearing spun.
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