Tapping noise from under bonnet

[810milky]

Hi
I wonder if anyone has any ideas on my problem.
I have a 2009 CLK A350, when I start up and pull away I sometimes hear a tapping sound from the engine area. This only seems to happen when I'm rolling, if I stop and rev up the noise isn't there. Also this problem is intermittent. (Sounds similar to a tappet noise)
Any advice would be helpful, thanks in advance.
Ps car has done 75k and runs smoothly.

 

[alecmascot]

Diesel or petrol ?

 

[810milky]

Diesel or petrol ?

Petrol

 

[W212250]

It could be the fuel vent valve.
If it is a constant tapping noise it could be. The 250cgi has it on the front right side under the bonnet