Yes, it is somewhat normal, Berk CBE + Berk HFC gets the burble too. 
My Mini Cooper does it too, check out the video on this one:
A couple links on articles, not sure how accurate they are:
Exhaust Burble Causes | eHow.com
eBay Guides - Exhaust Pipe Diameters
__________________
 370z Tour| Spt Pack|Navi|BERK CBE|Berk HFC's| Eibach Springs| Hotchkis Sway Bars|SPC Camber Arms|15mm Ichiba Wheel Spacers| 19"Factory Nismo Wheels-Matt Black|Stop Tech Rotors|TWM Shifter|3M Tint|Black Fangs|Black Rear Valance|JDM Fog Light| MY BAND- www.myspace.com/petroleum
|