What does this vacuum operated exhaust valve do? Can I unhook it? It's makeing a rattling noise.

Think this is a heat riser for cold weather operation. I'd guess the vacuum closes it and it would fail to the open position so if it quiets it down with the vacuum unhooked, do it.

It blocks the passenger side manifold so that the exhaust has to go through the crossover under the manifold to warm it up.