Very quiet actually. Have standard centre box and it doesn't make much extra noise at all. I replaced it as my standard back-box outer skin was corroded so got a cheap bargain Irmscher to replace it. Have been advised NOT to get a remus centre section as they are straight through and reverberate quite a bit & make it noisy outside & inside.
I'm on the lookout for a centre section with a small silencer in which will increase the noise and output overall but keep it generally quiet, esp on a run: this is my daily driver so didn't want it too noisy anyway. Plus I work shifts so coming home 2/3/4am with a noisy car is not good and didn't want that.
Haven't got any side pics, but it doesn't stick out too much if that's what you were worried about. You can hear it a bit, but not a great deal as you'd expect to be honest.