84 Suburban Idle issue

The hissing noise is a vacuum leak. Low voltage is from your low idle.

I watched your second video and I believe part of the problem is how you are starting the vehicle. With a carbureted engine you can NOT just turn the key and have it start you first have to hit the gas pedal at least once in order for the choke to set and the fast idle to engage as well as priming the engine. That said you also have to be sure the choke is set up correctly, I see that you have an Eldebrock with an electric choke on it which is a good carburetor BTW but if it’s not set up and operated correctly the engine will not perform as expected. Carbureted engines are a word of difference away from fuel injected vehicles and as such require that you modify your driving habits accordingly for best results.

I honestly don’t think I can properly convey carburetor tuning techniques via this forum so I would recommend you take it somewhere to have the carburetor adjusted should hitting the gas before you start the engine not solve the problem.