So I needed petrol and filled up with 97 ron instead of the usual 95. I haven’t heard the noise since. Must be some kind of pinking although wouldn’t have thought that would occur on a well serviced 3 and a half year old engine.
This comment brings to mind a situation, I had some 20 odd years ago whilst running a new Landrover Freelander.
I had similar noises, which I reported to my dealer. He asked what fuel was I using? To which I replied Supermarket fuel.
His advice was to try a tankfull of branded fuel ie Shell, Esso or whatever.
The noise went away and I have never used Supermarket fuel since.
Fuel consumption was also improved, which negated the slight increase in cost.