97 E420 119 engine, 105k miles

problems:
1. at idle or moving, there is a tapping/ticking noise that seems to be coming from the lower half of the engine (rod bearing?). happens at P, N or D. The tapping gets quicker when I shift to N/D than at P, and the frequency increases as I give more gas. What is the problem and how urgently should this be addressed?

2. Car has a rough idle (at D or P, but worse in D). At P rpm is a bit higher but at D when the rpm is at about 700, the rpm would drop to 400-500 and then the car would get it up to 700 again.. this happens every 15-20 seconds. Car doesn't die though. No engine malfunction codes.