It can be loud in my opinion, but then noise is always subjective to the listener(s). Definitely not as loud as a Monza or Ansa system, but louder than stock.
Before buying, try to find a local club event and someone will surely have a set installed. At the Triumph day in Brookline, MA this past weekend, several guys asked the car several spots away from me to start up so they could hear his Bell system. Then they asked to hear my stock system with headers.
Some liked his and some liked mine. All a matter of taste and hearing the exhaust on the same car as yours for yourself is always the best way to tell BEFORE buying.
Also, make sure that you find out how old the system is. They seem to get a bit louder with age.