The second connection is for the black tank flush. I have two black tanks so I have three connections. On mine the middle one is for the fresh water tank fill/water to the MH, the one on the right is the flush for the main black tank, and the one on the left is for rear black tank. When you hook up to the black tank flush line and turn on the water, it does make a humming noise. My guess would be that you are hooking up to the tank flush, and the water is going into your black holding tank. If you leave that on without the drain valve open you can overflow the black tank fairly quick, which will create a mess. Check the level on your black tank to see if you added water to it already.
It sounds like you are hooked up to the black tank flush. You might want to move the hose to the other connection and see what happens. The valve has two positions "city water" to supply the MH or "tank fill" to fill the holding tank. Just make sure you don't let the water stay on very long until you know where it is going. You can look at the fresh water holding tank to see if the level is rising. If not turn it off as you may be filling the black tank.
Mike & Charlotte
2014 Newmar Canyon Star 3610
Orange County, California