[da-burb]

That is a heat riser valve. First off, does yours still rotate? They are known to rust in place. You can wire it in the open position if it is stuck closed. What it does is force hot exhaust gasses through the intake system to keep it from freezing up in cold weather. As your engine warms up the thermostatic spring is supposed to allow the gate to open. It will also open under higher rpm's as you drive. They really don't make any noise. I am guessing that one of 2 things has happened. 1) it is stuck shut and your exhaust is restricted to the point of choking the engine to death, or 2) the gasket is out between the intake and exhaust manifolds and you are hearing the exhaust leak. My disclaimer....I haven't ever worked on a 6 cylinder and I am thinking that I see a gasket between the 2 manifolds in your picture above.