I've just started having this issue, I'll call it a rattle. Worst when no pressure on accelerator and in drive with a/c on, maybe 650 rpm idle. As I put it in park (very slight increase in RPM)the noise decreases. decreases again when the a/c turned off. At any engine RPM above 750 rpm right up to 3000 it is normal. I've done the long screwdriver to ear trick, but nothing stands out, In general it sounds louder on the Exhaust / turbo area. The head was off and completely reconditioned after an "ice plug" at the rear let go and spewed coolant out 6 months ago. most of the serpentine belt stuff, tensioner etc was examined and as needed replaced back then. Its 13 years old. 134,000km. And its had the Alternator replaced since the head repair But still on all the original exhaust (I'm suspicious of that). Weirdly, after I take a few things into the house, and come back, start it to move further into the garage - (2 minutes). Start, put in Drive, let it idle forward a meter, silent, no out of place sounds whatsoever. Apart from the above it is making good power, and the Turbo boost is onstream as needed or provoked. just the problem you don't want to demonstrate to a mechanic because sometimes its silent, sometimes not. https://photos.app.goo.gl/AAykKf8VDKQngnZcA