Tapping from 97 W202

[Allanmac]

I have posted this problem on a couple of forums in the UK but not found an answer, hoping someone here could help.....

I have a 97 W202 C180. Mileage is 105000 and it has a full service history.
A few weeks ago, it develpoed a tapping noise that can only be heard inside the car with the engine running. Tapping is, or seems to be once every revolution and is associated with a hesitant pick up when i try to pull away.
This tapping is not there all the time and when it isnt there, the car runs perfectly. I noticed fuel consumption is not as good as it used to be.
I have changed plugs and wires, coils, MAF and lambda ( oxygen) sensor. When I change something, it seems to run fine for a day or so, then the tapping comes back. I am convinced this is fuel or ignition related, not mechanical as the engine sounds sweet from under the hood, its only when you get in the car you hear it. Put your head in the trunk and you hear it there too, stand up, and its gone.

Help!! any ideas?

Allan