They get "adjusted" by using Mobil-1 5W-40 or 15W-50 synthetic oil, which often reduces or eliminates the 'tapping', if you have that problem. If that doesn't cure it, you can replace the lifters but they're not cheap.
If you're a new 1987 300D owner, read this:
http://www.w124performance.com/docs/mb/articles/124.1x3_buying_tips.txt
