Today was our first day out this year. The boat ran great for a few hours. We stopped for a bit to let the kids swim and have a snack. When I started it back up, there was a strange air sucking sound that seemed to be coming from the air intake - kind of like something was trying to suck in air but was restricted, or something that normally sucks in air but is muffled had the muffler removed. Also, if it didn't stall right after cranking, it would stall after giving it a little throttle. I took off the plastic cover and the flame arrester and tried cranking it. It made the same sound and wouldn't run long enough for me to go back to the engine after cranking it to inspect better. With it off, I opened the throttle all the way to make sure the valve inside was opening properly. Then, soon afterward (maybe the next crank) the sound was gone and it ran fine. I put things back together, and we probably ran for another 45 minutes or so without any problems.

Any ideas what that sound could have been?