I'm such an idiot. I didn't have a gamecube controller to test it on, and I 
didn't 
check my code.

Yes, it turns on rumble on the GC pad every frame of every game and every frame 
of 
every menu, regardless of what is happening.

Feel free to laugh at this code snippet (which is called every frame):

static void updateRumble() {
        bool r = false;
        if (InMenu) r = (menuRumbleCount > 0);
        else r = cartridgeRumble || (gameRumbleCount > 0) || (menuRumbleCount > 0);

#ifdef HW_RVL
        // Rumble wii remote 0
        WPAD_Rumble(0, r);
#endif
        PAD_ControlMotor(PAD_CHAN0, PAD_MOTOR_RUMBLE);
}

Note the constant PAD_MOTOR_RUMBLE instead of an IF statement based on the 
value of 
"r".

This bug only affects Gamecube controllers, so as a workaround you can unplug 
the 
Gamecube controller. I'll go and fix it now.

Issue 79 has been merged into this issue.

Issue 81 has been merged into this issue.

Thanks very much, works fine now! :)

:-)