It seems this gauge is a basic vacuum gauge. Reading in the red means a vacuum leak (low vacuum). I suspect the hissing you hear is exactly that, probably the hoses in the idle air circuit, but I would in any event remove the air cleaner and try to find the source of the hissing, and thus the leak.

Note your airflow meter will be incorrectly measuring airflow while the leak is providing an unmetered source of intake air. This will upset your mixture.

Steve