the 5.0 in my truck make a knocking sound sometimes at low rpms when warmed up. Also the oil pressure drops at idle sometimes when warm and at idle as well. It will go back up when I rev the motor up. I really need this engine to make it through the winter and there are hardly any mods on it. 160k though. Oil always changed every 3000. It sounds like a vibration in the truck that only happens when i rev it up and it then comes down and it starts at like 1000 rpm.