^^ not sure I would exactly agree with everything this article says, but the principles are good so definitely worth the link..

 

yup, it's engine braking. Totally normal if you're in 2nd or 3rd gear at 30mph in this car. I am not sure what the 1st gear top speed but 30 mph should be possible not not recommended. You will jerk your passenger's head like hell.
Also, the G has pretty high torque number at relatively low rpm. When you downshift, try to rev match it or else your clutch can wear out faster...not to mention your passenger definitely won't sit in your car again.