I had the same thing happening on my '89 190E 2.6. I noticed it in the spring and thought it might be valve train noise. A few months later my timing chain broke.
It may have been the chain. I'll never know. But, I just finished putting in new main and rod journals tonight and I get to pick up my cylinder head from the machine shop tomorrow (today?
If your chain has not been replaced, do it now. My car had 220k. Not far from yours.