The lifters do tap but the sound should deminish with oil pressure increase. You could try some SeaFoam for a few miles and then change the oil and filter. Search for past users and results. To replace the lifters, remove the top cam. Try to find "good" used as the new INA are expensive and may fail. You can test each lifter once removed. Post for more information.
If the noise is not coming from the top of the engine, it may be the rod bearings. After you use SeaFoam, look carefully at the oil and check for shavings of metal. Try to filter your old oil with cheese cloth or an old pair of nylons. Rod bearings should be changed at around 110 to 125K miles.
Another interesting link:
http://forums.rennlist.com/rennforums/924-931-944-951-968-forum/350563-knocking-from-cam-tower.html
GL
John_AZ
1988 924S + 1987 924S