Well I filled up with 95 the other day and under light accelleration up hill I thought I heard a similar noise...

I doubt if it is audible engine knock/detonation, it may be incorrect gear selection and thus vibrational load from the gearbox/torque converter, it was pretty faint and I have only heard it once. I will be calling the dealership later in the week when I have a free moment to ask the question. I think it may be worthwhile contacting the dealership NOW and booking it in for its 1000km check up and noting the problem...