noisy exhaust on mercury 60 hp ob

[fletcher]

hi all,took boat out yesterday.everything was fine till the ob started to sound noisy.so i returned to shore and took boat out of water,flushed it through.the 'pee' pipe was fine.but the exhaust area where the prop is was sounding noisy than normal.the noise was like a rust hole you would get on your car exhaust.any ideas as to what may have gone wrong with this ? thanks.