Peter beat me to it. There is a perforated open pipe down the middle which is wrapped or lagged with sound absorbing 'wool' probably glass fibre. This lagging tends to blow out over time, and becomes less absorbing allowing more noise out. There are a couple of ways of dealing with this, the easiest is to take the silencer to a small exhaust company and ask them to cut the end off and re-pack it, or just get another one made, perhaps slightly longer if there is room, but for goodness sake don't start getting baffles put in or your performance will suffer. Or, you could buy a ready made silencer from someone like 'Car Builder Solutions', just google them if you don't know who they are.
