Some polite ways of introducing complaints
Sorry, but you are shouting at me. (It is very informal) I am sorry to say this, but your dog was barking the whole night. I am sorry to bring this up, but your son has broken my window. I am sorry to have to say this, but your music is too loud. I am really sorry to trouble you, but I have to make a complaint about the watch you sold me last week. I regret to bring to your notice that the typewriters manufactured by you make a lot of noise. (It is normally used in formal written complaints)