Just got a F206, with merc 350 ( 260hp ) and I believe all stock thru hall. Has internal flappers and rubber flapper on the tips. Got an awful random knock at low rpm. Shoved a stick up the exhaust and noise was gone. Odd thing is it is only one side (driver side). Pulled the rubber exhaust hose and internal flapper looked new in there. Shiny stainless with rubber all around. Flapper is in the hose about 10 inches.

Could there be something wrong with the motor on this side messing with the flow of exhaust?

How do/can u get the flapper out of the rubber hose? Special tool? Looks like there is a section of thin metal pipe housing the flapper.

Any ideas or advice would be appreciated.

some flappers are on welded in pins, some pins are held in with exhaust hose. pertty much gotta take out tip to deal with it. unless taking off the hose shows you the pin. sometines, quite often actually, you hafta either take off tip or exhaust riser to get hose out. they ain't real bendy.

It's normal for those types of flappers to knock while sitting there at idle.

The internal flappers are usually connected to the exhaust tip. Many of which are welded in. That sound while annoying is pretty normal at idle. It literally sounds like someone's tapping on the rocker covers with a hammer and it's completely random because the exhaust pressure varies between the exhaust and water being ejected and the metal flapper is banging against the inside of the tip.