interesting to see how this thing finally found its spectators ..
I had the belt tensioner replaced under warranty by the dealer and took home my old piece. ( I guess they didn't replace the idler pulley, thoug).
I just now went to look for my old part inside the big box of all my toys, looked a bit more attentively at it (which I hadn't done till now) and took a pic
just to make things definately clear, I was talking of the LEFT bank
Having it at hand is now clear and sure that it was because of it being worn
If you want to check the state of yours, just loosen your compressor belt so that you can take the tensioner pully with your hand.
then, try to mobve it quickly up and down.
if you hear that knocking noise ... it is it !