A loud sighing noise comes from our AC Unit. More so when it is extreamly cold or hot. What is this noise?

Heatpump? To nondescript for me to guess.

A loud sighing noise comes from our AC Unit. Moreso when it is extreamly cold or hot. What is this noise?
Yes, Electric heat pump. The unit outside makes a loud sighing noise like it is being overworked. The emergency heat is not on at this time.

First thing I would check is airflow in the inside and outside units the head pressure may be getting to high due to low indor airflow. If it only happens when the unit first turns on or off that is also a common complaint when the reversing valve changes refrigerant direction flow.