I kinda thought it was a 32 with silent choice. Hence my questioning. ;)

Anyways, it is the nature of the beast. All 32's do this. It is because the 32's sit so low in the back, that the exhaust tips are trapped in an air pocket underwater under the swim platform. It is not the exhaust flappers. What you are feeling/hearing is the exhaust pulses trying to escape from the sealed pocket by blowing past the swimplatform at the water line. It only does it at idle or while at idleing speed and with the silent choice in loud mode. You have to go fast enough to break the sealed air pocket to get the rattleing to go away. Or run in quiet mode at idle. Mine sounds like the hole boat is about to come apart, so I ALWAYS switch to quiet and exhaust thru the drives. Nothing you can do about it.

Now if it happens while in quiet mode, then it is the valves in the y-pipe. My last 27 used to have a rattle at idle. Sounded like a lifter ticking. Valves were new too.

Rattle
 

Mine did this. Just only the flappers in my case.