Evenin' again...
OK...what you say makes sense, as the torque of the engine either on acceleration or deceleration the sourse of that noise it loaded. So movement that contributes to noise is muted.
You need to think of a component (like the steel disc that's affixed to the left bank timing belt pulley). I havent a clue where precisely that noise is coming from, just by listening to the vid. But I make mention of the timing belt guide simply as a thought, you can imagine if that 8mm guide bolt were to come loose, then that belt guide disc would rattle against the alloy gear, except when the engine was under load (the belt would mute it).
Other possibilities are, loose exhaust heat shield, stone or a detached nut caught in the header heat shield, something broken off in the exhaust header pipe collector (tho these last suggestion are more likely to produce rattles when the engine is loaded), damaged alternator drive coupling, or any component thats driven, that if it either damaged or loose, might cause noise when not under load.
Dunno if your going to Hepburn tomorrow, but I will put a stethescope with a probe in my pannier tomorrow. I am meeting a Mate in Gisborne for a coffee at 10, so if the weather gods are kind, will pop over to the pub at lunchtime.
Sorry but best I can offer on an iphone.
Cheers.