There's an upper tensioner on both sides behind the black cover. Mine did it on the left, and the right was silent. The vicious brief rattle is very well documented on the early models, and there's tsb's for it. After looking on other forums for ages there's only a few cases I've found where they have this problem as well, it's like a quieter rattle that gets louder over a few seconds then stops, then a few seconds later does the same, on mine it did exactly like it does in your video, sometimes wouldn't do it at all, and then other times when it did it it would last around a minute like you describe, it would carry on for a couple of times after the high idle after cold start.
I didn't really worry to much as when it's hot it's fine, and like i said changing the oil seems to have sorted it anyway. The tensioners are known to be dodgy on these and people who have had them replaced have reported that noises have come back. So for me I won't worry until something major goes obviously or a cat goes and I have to get the engine or box out anyway and then I'll do all the chains and tensioners at the same time. It would be good to test it once the engines warm/hot, turn it off and start it again after a couple of minutes or so, if you get any rattling at that point then it's a serious issue!
I understand the principle of how the tensioner works off oil pressure, but I don't really understand exactly how this noise works, as it seems to come and go, so whether it's related to oil pressure or just not enough oil reaching the chain drive? The brief loud rattle problem is just pressure where it drains out and has to come back upto pressure when started but this is different, I think if it was pressure related it should either rattle or not, and not come and go! I'd love to get an oil pressure gauge on one when it doing it and see if the oil pressure is actually fluctuating. All I know is oil fixed it! 0w40 is thinner when cold than 5w or 10w, so my theory is thinner oil means oil travels easier through the engine and pressure/flow get to the top of the engine quicker.