Gethen

When my E320 stops at traffic light, often I hear quiet tap...tap-tap... sequence once or twice, very similar to the sound of water dripping on the roof.
Didn't notice this before, I wonder if a pump is getting ready to quit, or if something is up with brakes.

lkchris

Take a look at serpentine belt tensioner.

RichardM98

There's also a purge valve located just behind the window washer reservoir labeled 'MOT' which makes a clicking sound when activated. Some have said they can hear it in the car. I can only hear mine if I'm next to the fender with the hood open.

Good luck.

Gethen

I'll check it out, thanks!

RichardM98

Forgot to mention earlier but whenever you hear an unknown sound rev the engine a little and listen to see if the frequency or pitch increases. That may help identify possible source.