What could be causing knocking sound when idling?

NO , it doesn't have lifters. It could be a bad timing belt tensioner (serious) , accessory drive belt tensioner (less serious) or even a bad crank pulley. More description of conditions may help narrow the guesses down. Does it make the noise worse when cold start-up or after it's been running a while? With A/C on or off or doesn't make a difference? IF you very gradually increase RPMs , at what engine speed does the 'knock' go away , or does it just become less noticeable , but still there? Does corn make good gas?