I currently have the same sort of noise in my VY, anytime I ring it out over say 5500-6000rpm, just before the gear change, there is kind of an "almost" rattle noise, almost sounds like something metallic in the left hand exhaust bouncing around, I am currently putting in a rising rate fuel reg to do away with the original and to where I will bump up the pressure slightly to make sure it just isn't leaning out and having a slight ping at higher rpms under load ( I run 98 octane with a +10 Booster so fuel should be mint ), a new PCM with A/M tune has gone in and the sound is still there, my fuel reg may have some issues as after the car has been sitting overnight it will take 2 turns to start ( not every time, sometimes first pop ) but if I drive it and stop then start ( at any temp ) it will start instantly every time ( vr vr vroom ) , I'm guessing the fuel pressure is being released though a worn regulator, if that doesn't resolve it I may bring it down to the timing chain getting loose/worn or valve train noise
