the question of changing ethernet cables

Hi everyone, I am also new to the site and I have followed many conversations learning much. I have ethernet cables running from the router to the streamer (PS Audio). However, I have a fiberglass cable running from the street input to my main box. That should mean from what I have followed here that I am not bringing any noise into the system from outside. I too, have thought about changing the internal cables and/or employing an isolator of sorts. However, in my case, I am not sure that I need to. Any suggestions would be welcome.