When you say it stops, are you saying the pump stops or are you saying the pump continues to run, but the hissing stops? If the latter, I would agree with cboss that your diverter valve is in the winterization/antifreeze suction position. Flip the switch on the valve and see what happens.
2016 Rockwood Windjammer 3029W Diamond
2015 Chevy 3500HD LTZ 6.0 Crew Cab 4x4 Long Bed 4.10:1 SRW