Exhaust manifold valve

I have a Buick Special Convertible from 1957 with some noise (sounds like metal against metal) when the engine gets warmer. Some times I can run 20 miles - other times 40 miles before the noise starts low and then gets higher. The noise stops when I give full gas or no gas, but is constant by cruising.

I have been told that it might be the heat control valve with the bi-metal thermostat, which is broken.

Is there somebody who can advice me?

Best regards

Per

Wire it up and see if it still make the noise.