I currently have a 2002 496 mag ho and last time using the boat all was well. Took it out tonight and when I first started it it made an occasional sound like it was real easing air with every couple revolutions of the crank, kind of like pulling the air release on a compressor. After a quick run it stopped but if you give it a quick stab of the throttle it makes that noise for a second after it revs down every time. Motor idles perfect otherwise runs as strong as ever just don't want to damage anything using it anymore any and all help would be greatly apprecited..

Exhaust leak, possibly a blown manifold/header gasket?

IAC muffler missing or somewhat disintegrated or ingested in the cavity that it sits in. Also clean the flame arrestor from the inside out after you pull it off. I have never seen a boat with a clean flame arrestor when it came in for service.

My former boat did this and never knew why. A thread was created on here a few months ago with video of engine running. Answer was replace the IAC. Wish I would of known.

Its the sound of the IAC adjusting and causing air to be sucked in.