1) check your oil, if its low (like barely on the dipstick, then assume its a rod knock)
2) If you have plenty of oil, drain your oil into a CLEAN oil pan an check with a MAGNET for metal shavings.
3) pull the oil filter off, and cut it open an inspect for metal shavings.
4) Pull lower pan off, and check for metal shavings

let the car warm up, and see if the knock goes away.