Hissing Sound?

If it's that little round thing inside the air cleaner that's about the diameter of a quarter & has 2 hoses hooked to it, I think it's there to measure intake air temperature & help determine when to open & close the heat stove attached to the air cleaner. It has a bi-metal spring that controls vacuum to the valve on the intake of the air cleaner. That valve is what controls the heated air coming from the exhaust manifold through that big (2" or so) tube up to the air cleaner. I could be completely wrong though.