How old is the car battery? If it is the original, replace it now.
I am not familiar with W163 cars. I had similar sound (though slower,
about 1 tap every 2-3 seconds) on my 96 C280 (W202) from around the
motor driven throttle valve (I suspected) many years ago. I never
figured out what it was but the sound disappeared after battery was
changed. It may be just a coincident but worth checking.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.