[jlobon]

At least in mine (40 express model year 2000)the engine exhaust at idle is relieve thru a couple of dedicated openings under each side of the platform at the water level, however if the boat is listed (inclined) to one side or the other ( for any number of reasons), even if not very noticeable, the openings on the listing side might be completely submerged while the ones on the other side might be completely above the water line and that might explain the difference between the engine perceived behavior / noise.