Checking for water system leaks in a Northern Lite

Water heater pressure relief valve (the one you see when you drop the water heater door on the outside) is weak and releasing a little. It happens. It will also do it if there is a calcium buildup.

Or on the inlet side (not the pressurized side) of the pump the hose is just loose enough so it emits a little air into the system. That is a common one folks often miss.