Over the past year, DS (almost 7) has begun sniffing. It started as an occasional thing but since his return from almost 3 weeks with his Dad, he's doing it ALL the time, sometimes between each breath. I don't know how he's actually managing to breathe properly. It's really loud and sounds unpleasant.
I'm torn between ignoring it in the hope that it'll go away (but it's got worse over the year) and telling him to stop (it's not nice for other people to hear and his friend has already commented on it - it won't go down well at school). I've tried various things - asking him outright to stop, gently tapping him to make him aware he's doing it without drawing attention to it, tapping my nose etc as well as shouting when I lose my patience 
I really don't know what to do to help him. He claims he doesn't know why he does it (which is probably true) so I'm at a loss. Any suggestions?