I just had this exact problem. It can be one of three things: air in the water system, a failed or failing pump or a leak in a faucet or other part of the water system. Most likely it is air in the system and can occur when you run low on the water supply or just after coming out of winterization. It can also occur when you switch from using city water to your water tank.
Here is the fix that worked for me, fill the water tank until it overflows at the fill spout. Water should pour out of the air port at the fill spout. Now turn on all faucets and the toilet water supply and continue running until the spurting stops. Be sure you are running all faucets: hot and cold, inside and outside. This may take several minutes of running. Once the spurting stops refill your water supply and test, the pump should now stop running.
Paul and Janet Braun
2003 Toyota 4Runner V8 now 2012 Toyota Sequoia V8
Escape 19' 2010 now 2014 Escape 21'