I have a 2012 Mercury 383 Magnum with the same problem. I was just letting the engine warm up at the dock and it just started. I had the engine tuned up with new ignition parts. Runs perfect except that same loud sucking sound. It does it all the time cold or hot. Motor has 150 hours on it. throttle plate is opening and closing fine. I saw on a car forum a similar complaint and a suggestion was the "IAC is stuck open all the way" what does that mean and do marine engines have an IAC?

Thanks