Engine tapping noise when idling - 2013 Mercedes-Benz GLK 350 4Matic

Injectors do make quite a racket but mostly they are so well insulated they can't be heard inside the car and in the engine compartment the noise just becomes part of the general engine noise.
I have noticed a few cars where the use of semi-rigid (nylon) fuel supply and return pipes transmit the noise to the bulkhead/firewall which magnifies the sound.

That could be a whole host of things.If the tapping increases in speed with rpm you are looking possibly internal but if not the problem could be something loose like the exhaust pipe.

Does the noise sound like its coming out of the engine or transimission because I can't help,you if I'm not there but does it sound like a tapping noise a lifter or or does it sound like a idle pulley get back I'll help you good lucl

Usually, this would indicate sticking valve lifters !!

for v-6....3.9L

These engines are equipped with hydraulic lash adjusters designed to maintain zero lash at all times.

for 4 cyl...2.5 sohc
Valve Lash
Non-adjustable Dry Lash 0.024-0.060 inch.

for v8,...5.2---These engines use hydraulic lifters. No provision for adjustment is provide

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Sounds like you have a sticking valve

Remove the valve cover and rotate the engine and check to see if all the valves are opening the same amount. If not, you have either a bad lifter, bent pushrod, or cam going flat. Check your valve lash with the valves closed. You should be at 0 lash at all of the rocker arms.

That noise is most likely cam follower noise ( formally know as lifters), this is common but no normal, it should disappear after a few seconds when the engine is started, talk to the dealer about the noise, he may be able to help even if you are out of warranty.

I assume you mean low idle speed 500 rpm. Genarally a tapping noise comes from the valve train, and a knocking noise comes from the crankshaft connecting rods. The way that a mechanic will narrow it down is to use a mechanics stethoscope. You can get one pretty cheaply at an auto parts store. Put the metal end of the scope on a part of the engine and listen for noise. On the other hand, if your car is idling at 5000 rpm, it would also be making ALOT of noise.