The burble comes from the additional components and, more importantly, the tune. Good read on the birth of burble here:
https://drivetribe.com/p/exhaust-pops-bangs-explained-by-AC7bCQ84SQy9_WKJLKuUgg?iid=IIT-5FF_SnaLaotxZh_uAw
There is also another thread on the differences on the B v GT...