The heat pump condenser is not required full time in the heating mode. It is discharging cold to the condenser coil (which has really become the evaporator coil) and collecting heat to bring in the house.
Quite often the condenser unit may ice up, which brings its efficiency to almost zero. Some units have resistance electric defrosters that kick on while others use a blast of hot gasses to defrost it. This hot gas defrost will sound like the unit is discharging, it gives off a big spewing sound very quickly.
If neither of my diagnosees appear to be working and the problem persists, come back and we will try again.