Some can be pretty damn loud.
All fans are more quiet when they're hooked up to ducting, especially insulated ducting, so keep that in mind when you first plug your next one in...
My old CAP fan was super loud. It was a 6" high output. I'm using a 4" sunleaves windtunnel right row that's super quiet...like 1/4 the CAP, but also about 1/2 the airflow. I can only hear the air moving, pretty much.
Some people use speed adjusters to turn their fans down a little and reduce noise. There's been trouble in the past with some speed controllers with certain fans...but they can work well and are another option if noise becomes a problem. Another solution is to build a small box that holds/insulates the fan. You can use wood or a rubbermaid.