my 5 year old daughter does the same thing and it annoys me to no end as well. All the time, when she's watching tv or eating or drinking. Playing games on my iPad. She also like hums? Not like humming not whimpering it's hard to explain. I feel bad because i don't think she can help it but omg.
I would like to hear how your son is doing now. My son does the same exact thing. He has also done this even when he was a baby and we thought he was constipated (he's almost 3 now and its not because of constipation.) He is a stout build and has a little pot belly wondering if caused by this activity?
Just a quick note to say hi!