I don't know if it's a coincidence, but the intermittent tapping started a day after I started driving in S mode (fulltime).This was about a week ago.
Two days back, I put the transmission back to E mode. Although the tapping was still there initially, it seems to have disappeared.
Btw, the tapping seems to come from the engine bay near the passenger side firewall.
Of course it can only be heard from inside while accelerating.
With the bonnet open in P or N, there is no tapping.
Tapping seems to correspond with engine speed when it comes.
Babybenz 1991 190e 1.8