Don't get too upset about the tapping. MB redesigned the lifter several times and never was able to eliminate it entirely. Using good synthetic oil seems to help a LOT but doesn't work in 100% of cases. If your lifters are original, you may have a couple that are bad - they DO fail, they don't last forever. There is a procedure in the OM602/603 factory engine manual on how to test them. On my car, at 230kmi, I had 5 that tested bad out of 12. But in most cases synthetics fix the noise. IMO, your gasket problem is the rare exception for the lifter noise...!