There is a slack side but if the pulley is tight, you can't twist the belt through more than 90deg, and the rattling was definatley the heat shield, id say the belt is ok, you would know about it if it wasn't.
Still say that it Looks a bit slack on that vid, just the way the lower part of the belt moves but you can't tell how much force you are applying.
Glad its not the belt snd sorry for freaking you out, my bad.
I would certainty be replacing that tensioner though.