[Familyman 90]

On the models with the filler on the right the elevated tapping noise alone does not automatically indicate a problem. You will also feel it with your palm over the cap, and if the engine is warm you may also see slight oil vapour being blown out the open cap.

My V70, right hand filler, does it and there is no definitely, positively, absolutely no leak. I know this for certain. I'm retired, home all the time, and I will happily make my V70 available for you to come round and inspect for yourself (I'll get the kettle on). You can't argue with bits of metal you can see and touch.

It may well be that yours or another car has the problem. However, it is indisputable that models with the filler on the right are naturally noisier with the cap off simply because they are next to the pump and have nothing in the way of distance or baffles to damp the natural noise, unlike the more common filler-on-the left models. The noise on its own is not indicative of a leak.

There is even a technical bulletin on the subject, you might find it an interesting read.

You might also wish to consider that its a rotary pump, and not a plunger type, and doesnt actually tap or provide a pressure pulse anyway.