Supposedly it'll break in and quiet down after about 1000 miles. You can be kind to your neighbors and take a nice long road trip.

Ivory Coupe 5AT
Z tube, K&N drop in.
Fujitsubo Catback

 

After having 4 different aftermarket exhausts on the same car over 10 years, I believe you will find that the measured noise level increases with age as the sound deading materials break up with age and blow out the exhaust pipe.

Of course the noise frequency spectrum changes as the system warms/heats up.