As the ENT surgeon I can help you in this that most common cause of foul-smelling is: 1: poor oral hygine 2: chr sinus infection Are r experiencing normal smell as foul smells Do u have frequent episode of or history of allergic rhinitis But even after these two things r ruled out then it is known as cacosmia which is related to olfactory system. For further guidance and management free to consult with detail history