It might be the nasal grunting sound. As baby is only few months old, they still trynna get use to breathing through nose and not mouth. It can also be that their nose passageway is small hence sometimes you will hear like them like snore, some little noises.
My second born had this kind of noise and I requested for nose check, saline drop and to clean the dry mucus to help her breathe better. So far so good.
Yu Shan