in the classroom ,when students are too noisy ,they always hear a "sh" sound,which means they should be quiet. It will work at the very first time. as time goes by ,people will feel it annoying,especially when the "sh" sound lasts a long time. If i am thinking about a problem, it will disturb me ,and i have to quit. Hence , i don't know why ,the continous sound of this kind makes people annoyed.as for me ,this is sort of noise,too.