The lifters are hydraulic in that the oil is designed to pump them up so that there is no clearance. If the lifter fails to fully pump up due to clogging of internal passages, it is referred to as "collapsed" or "partially collapsed". This is when you will hear the tap because there will be clearance between the lifter and what I believe to be the rocker arm on the 603. On pushrod engines, the clearance is between the cam and the pushrod.