It's domainance. The easiest way to control it is to whip out a firm "NO" when it happens (WHILE it's happening, even a few seconds after it stops does no good). Maybe keep a leash on and when they do it, step on the leash and pull back with it.
I've also read to give into the dominance. If the dog thinks they're dominant, then pet them first, let them out first, feed them first, etc. I'm not sure if that is a perfect treatment, but if the dog hums to prove this dominance they want, then maybe letting them have it may help things some.