Thanks for your thoughts, guys.
Steve - Had a friend with me to check while I drove and it's not a ground connection coz it's not from the speakers and definitely from the engine bay.
JDUB - Interesting thought, coz I saw a post regarding the tensioner shock that said "If I remember correctly, it takes up / damps quick motion and directional changes when you put your foot down on the loud pedal or when you let off suddenly" by Jay Yambrovich a long time back.
It would make sense coz I only hear the tapping sound when accelerating or decelerating as I mentioned in my earlier post.
I'll check it now and let everyone know how it goes.]
Babybenz 1991 190e 1.8