help with unusual noise

what engine are we talking about?
Did you replace the oil pump for the exact same type that was in there before ? if you went from balanced to unbalanced, you need to block an oil way or you lose a lot of oil pressure at low revs.
What bearing clearances where chosen? Where they confirmed with plastigauge measurements?
This could be rod knock from excessive clearances and/or too low of an oil pressure.
As long as the engine oil is cold, idle oil pressure is large enough to cope with both issues.

there may be your issue with the King bearings in only STD size. Did you actually measure rod bearing clearance? You might have excessive bearing clearance.
King offers three sizes of bearings allowing to set 5 sizes. I also use them and always needed to mix sizes to get into the clearance tolerance range required.

King sells 3 sizes, STD, -0.025mm and + 0.025mm. This allows you to adjust the bearing gap in steps of 12.5µm as you can combine a STD and a smaller or a larger shell.