When shooting pool and especially against a player you do not know, it is proper to be quiet during your opponents shots. Many bad players use noise to their advantage to rattle your game so its best not to do the same to your opponent. In many cases people do not know they are being rude during a shot, obviously if the pool game is going on at a loud bar with music or some type of sports game is being played this can not be helped but it still does not give anyone an excuse to be loud during pool opponents shots.
The best pool etiquette when it comes to noise is not to speak to the player when they have bent over for their shot, keep quiet and still, and if you need to speak during your opponents shot then whisper to the person you wish to speak too.
The worst things to do is to talk to a player when they are bent over on the pool table about to shoot which may cause a break in concentration. Or in mid pool stroke to yell or make a loud noise before pool cue impact. Or the obviously talking loud while someone is shooting because again this may break the players concentration. All of these things usually are a sign of a player who is ignorant or purposely trying to rattle the opponent from shooting well either way its just dirty pool play.