It definitely didn't sound like engine knock. What concerns me is the initial rev up the sound didn't exist, it was only once the rpms were falling and you gave it multiple consecutive revs that the sound existed which usually indicates either something physically shifted (exhaust leak or something) OR oil starvation valve noise (which is what it sounded like to me personally).

If the sound comes back try have someone else reproduce the sound while you listen at the engine bay and vehicle sides to determine exactly where the sound originates.

 

I had my long distance drive to work today. Drove it in DS and got on the throttle quite a bit. No problems. Confusing, but I'm pleased that things seem okay.

 

The sound came back at high revs yesterday (this time at a higher RPM). Added fuel system cleaner. Went away again.

I'm going to take it to Infiniti.