315 HP 6 cylinder, funny noise, then smoke

After a 5 hour uneventful cruise at 3200 rpm, I was docking the new boat to me (2003 shamrock mackinaw) and when I put it in neutral in made a very strange noise. Sounded like a loud bilge pump sucking air. As soon as I put it in forward, or reverse, the sound went away. I got the boat docked and shut it down. There was a strong smell of clutch or breaks, and when I lifted the engine cover, there was smoke. Next day I checked transmission oil, and it was full, and clear, and did not smell. I started the engine and put it in and out of gear, and all was normal. No funny noises in gear or out.......any ideas?

thanks in advance.
steve

Maybe a long shot but I'd check the V-belts...especially if there is a dust trail (port - starboard) at the front of the engine...probably wouldn't hurt to check the intake plumbing as well...something may just have come loose...