You may have an air bubble in the hot water heater. If that's the problem, the air acts like an accumulator and the pump is trying to pressurize the tank.
Shut off the HW heater and open a HW faucet to let some cold water in the tank. After the water cools down, open the relief valve on the HW heater (outside) and see if you can purge the air.
You could also have a bad pressure switch on the pump. It can be adjusted, but it's tricky and best done by the dealer. My brand new Outback did that the first day at home. Had to make the 90 mile trip back and they just replaced the pump. Never had a problem after that.
2015 Jay Flight 32RLDS Elite
2012 Chevy 2500HD Crew Cab LTZ (6.6L Duramax/Allison)
Equalizer Hitch (1200# bars)