Noivern are an interesting case, actually. By and large, the majority of noivern subspecies are actually insectivorous, meaning they subsist entirely on insects and bug-type pokémon—hence why pokédex entries for noivern state that they use ultrasonic waves to detect and snatch prey in the darkness of moonless nights. However, many noivern are not obligate carnivores and can eat both berries and insects, and some subspecies are even frugivores in that they strongly prefer fruit over anything else.
So generally speaking, noivern as a whole are considered to be carnivorous, but admittedly, it is true that there are herbivorous variations of the species.
The Pokédex According to Bill 