That was my thought initially but for a modern engine it does sound a bit 'tappity' but as I don't know what it sounded like before I can't really help.

I had the same issue. Some 'mechanic' though it was engine knock - he was clearly wrong - and he worked for mazda!

Another chap thought it was tappets - plausible as it's from the top end of the engine - but this was not the case

Is it by any chance a lot worse when the engine is warm?

If it is - it is the 'butterly flaps' in your manifold. Their job is to regulate airflow and reduce emissions until a certain rev point.

Mazda use a plastic manifold and the only option you have is to replace the entire manifold.

It will do no harm to the engine at all - just slightly lower MPG (it decreased mine by around 5%)

Take it to a garage and ask them to see if the noise is from the manifold

As balleballe says take it to a garage. Diagnosing engine noises on a forum is a total waste of time. Add to that any 10 year old car will have the odd noise.