Quicksilvers

I have installed five AAM resonated and non resonated short tail exhausts and I have never heard about anyone complain about any buzzing,rattling, vibration etc. noises after the installation. There is definitely something loose somewhere obviously and/or rubbing against something. Trying to diagnose a noise on a forum will be hard and it is better off being diagnosed and look at in person. You should take your Z to a reputable exhaust shop and have them hear this noise in person and take the exhaust shop technician for a ride and give them a chance to hear the noise. The fix is probably an easy one you just have to take your Z to the right place to diagnose the noise the best of luck.