Mmmm, your car is a left hand-drive, right?
Well, my car is a C320 Right-hand drive, but uses a similiar engine, 3.2 V6. I also had an intermittent tappet sound (ever so faintly) on my left bank, away from the drivers side but at the rear of the engine, low down. The sound was only imminent once the engine is up to operating temperature and especially pronounced if I come to a rest.
I had it checked out and even though it sounds as if it is coming from the left bank, the dealership told me that it was an MOT valve situated furthest from the drivers side. Naturally, I argued that it could not be and they also told me that it was not present on the newer models due to the fact that they had a valve in between to equalize vacuum inside the engine and the other items requiring vacuum on idle.
To cut a long story short it makes a noise on the rear of the left bank, the bank furthest from the drivers side when warm as the valve opens and closes, its actually an echo in the cam box. All they did was fit a non-return vacuum valve and wallah!
Quiet as a Merc should be [
Worth checking out before you let them touch the engine. Apparently, only the V6 engines have these vacuum control units which sits against the firewall called MOT valves.