Dictionaries are pretty unhelpful on this one. The best explanation for a racket meaning a loud noise is that the word may be imitative of a clamourous sound. Not that convincing, really. It's first recorded use was in 1565, so it's been around a while. In the 17th century 'to racket' meant to have a good social life. Nice one. (Unless of course you meant 'constructing a raquet...')