Growl: this is the area that gives grind to guitar and basses, and particularly with distorted lower sounds, will need attention. This can not only make your mixes quieter, but also sound more muddy and less focussed. In the lower bass and sub-bass, these sounds can build up quickly, while not perceptibly adding an actual note for the listener. Rumble: anything in the bass frequencies that's a lower octave resonance rather than a fundamental note.
