Mercedes tapping noise???

[Hoosier]

I've had E-class, M-class, and C-class Benz in the last 3 years. There is a ticking that is normal to all. The key to the ticking is whether it changes with the engine rpm. These cars have a catalyst reclaimer valve. When the engine gets hot there is extra catalytic heat that need to be expelled. This valve opens and closes when needed to release that heat. When it is open it ticks and it does sound like a valve ticking. When the engine cools is stops ticking but again the real thing to watch out for is when you hear it ticking rev up the engine. If the ticking stays the same speed it is the normal valve working correctly. If the ticking increases speed with the rpms get this car in to the shop...or at least tell your father.