I did for the first time the other day on a very rapid deceleration followed by hard braking. Just the usual idiot cager that ran a stop sign. Scared the crap out of me as it's never done that before. I have heard that others get this on occassion as well but if you are getting it every time, I would say there is an issue.
If you want a little more sound, take your baffles out (easier said than done, unfortunately) and cut them. I started out with mine all the way out, but it's too choppy, so I cut the baffle down to 3/4 original length, sounded alright but I wanted more. So I cut the baffle in 1/2 and love the sound I have now. It's louder but not too loud (for me!).
Congrats on the new pipes!