Knocking sound when engine is idling

Have it scanned for codes. Check for vacuum leaks. Check oil pressure. You can rent or borrow an oil pressure gauge from a parts store. Remove the oil pressure sensor, on the left side of the engine near the oil filter and replace it with the oil pressure gauge. Start the engine and read the gauge at idle. It should be at least 30-35 PSI. If not you may have a rod knock and the engine will need to be rebuilt or replaced.