I had the low oil warning at idle but my truck was also rattling from the engine verifying that there was actually low oil pressure and not just a bad sensor, Before tearing into it I ordered oil pump timing chain and all associated gaskets, when i tore it down I found a brittle o-ring at the oil pump pickup to be the culprit, after repairing this, oil pressure was back to normal and even my rattle at cold startup was gone, this was my situation