My original post:
1997 S90
When you first start the car the lifters are noisy for about 3 seconds. Then they are quiet. It seems once the oil is up to temp one of the lifters is not getting enough oil flow or the pressure is not maintained. After I used a professional motor flush and filled with Castrol 20W-50 the results are still the same. Would a straight 50 weight motor oil help?

Solution:
After trying a 50wt oil as an experiment the tapping continued,perhaps even worse. At this point I thought the only solution was to replace all 24 lifters and buy all the special tools required.
I talked to a mechanic at a dealer and followed his suggestion. I switched to a 5W-30 penzoil full synthetic and the problem is gone completly.