my outdoor unit is running nonstop for atleast 4 days?

i have noticed that my outdoor unit has been running nonstop for at least 4 days.my house is heating fine and the thermostat is shutting off correctly when the correct temperture is reached. i called the heating and air guy on day one and he said not to worry about it "its probly in defrost mode." well a couple of days later its still running. the temperture outside has been in the upper 50's on average this week. i turned my thermostat off today to see what would happen. the unit continually ran. i had to kick the breaker to it for it to stop. the full unit(both inside and out) are less than 5 years old. its also emitting a loud humming noise that we can hear in the house, befor we couldnt hear it at all. does anyone know what the problem probly is? or what to do to fix it?