Is the light tapping noise At idling my lifters going bad? Est cost of repair?

Hello, When you hear a tapping from the engine, there are a few things that could be the cause: Lifters, exhaust leaks, piston slap, and detonation are the leading causes. Lifters, as you mentioned are the number one cause of tapping. The lifters in your engine are hydraulic, and tapping would indicate they are not holding oil pressure, or the camshaft or lifter has worn. The M273 engine in your vehicle is fitted with non-adjustable lifters, so when there is wear, the only way to repair is to change the lifters. There is a great chance that you have bad gas, and if you recently fueled the vehicle at a different location than normal, I would advise refilling at your normal fueling station at 1/2 tank to see if the noise subsides. Also, make sure your oil is up to Mercedes Benz standards, and full. If you still have the noise, I recommend having it diagnosed at the very least, but if the noise begins to be anything besides light, repairing the issue will help maintain a reliable and efficient engine.