Herb,
The OM603 engine are notoriously irritating to fill with coolant. What I do is drain the system totally, including the block drain. That way I know how much I need to add (10 quarts for my 1987 300D - same engine as yours). Jack up the right front corner of the car high in the air. Add as much as you can to the top of the bottle. Wait until the level stops slowly dropping - this takes 5-15 minutes, it is NOT a fast procedure! Squeeze the upper radiator hose a few times. You can tell when there is liquid present in it. Don't start the engine until you have liquid in the upper hose!!! This should get about 90% of the specified coolant in. THEN start the engine, when it warms up, the level will drop slightly and you can add the final quart or so. I think the problem is most people do not wait long enough. They pour it in until the bottle is full, then go start the engine.
Your "valve tap" is likely to be one of the hydraulic lifters. This is probably a freak coincidence and had nothing to do with the coolant flush or engine temps. The noise is VERY common on OM60x engines. The cure, 99% of the time, is synthetic oil. I finally convinced my sister to switch from Delo-400 to Mobil-1 (15W-50). My BIL called today and said the lifter noises went away in days, and he wished he had listened to me & switched to synthetic 2 years ago (when they bought the car). If you use synthetic oil already, or the noise doesn't go away within about 5kmi after switching, it could be a collapsed lifter (or several). This is entirely possible with 150-250kmi on the engine. My car had 5 bad at 225kmi and I replaced all 12 of them, now it is nearly silent at idle!
(I kept the 7 good used ones as spares for my other car.)
Regards,