I just bought a 1998 32' Fountain and at idle when the Corsa switchable exhaust is in the open position the back of the boat vibrates alot. Is it just the rubber flappers banging on the bottom of the transom? Someone told me it was the internal flappers in the tips that are causing the vibration because there isn't enough exhaust flowing at idle to keep them open. Any advice this is really annoying.

The vibration you are feeling is because the exhaust is trapped under the swim platform in an air pocket created from the platform being at water level and has to blow out from underneath it. Has nothing to do with the flappers. Just use your silent choice at idle to rid the "vibration".

I think F Mike & USA1 are right. My 24CS does the same thing with exhaust opened.