Phantom Knock

Thanks jaws.Would that piston rock go away after a minute or so?If the car sits overnight and you fire it,its loud when you start it.If you fire it up a few times it progressively gets quieter.I fired it 4 times the other day to listen to it,and by the 4th time it was pretty much gone.Engine didnt even have time to warm up the oil.but if it sits awhile it makes noise again.Im getting ready to pull it tomorro