A growl? Someting you feel or hear?
Like a vibration of sorts?
The noises all have a resonance, like someting at wheel speed,driveline speed or engine rpm. My helicopter buddy shared something interesting,put your head against the side window, or doorpost. Any vibration transmitted into the body is easier to identify by determining how it resonates. It will at least help you determine where its originating.
There is a tool called chassis ear, a set of pickups or microphones and can be switched on/off and fed into headphones. A handy tool.