Look it's easy to find out if it's air flow related or freon pumping through the lines. Heres what you do go outside and "hopefully" you have a disconnect on your outside condenser well flip that sucker off, if no disconnect well kill the breaker to the outside unit and then go back in and turn on the unit inside to the fan on position on the thermostat and see if you still hear the noise.
During the cold weather a heat pump does its best at 450-CFM per/ton airflow.
That is 1575-CFM in the heat pump mode.
It also needs to be tolerable at that airflow level.
Everything to do with the airflow & noise, needs to be thoroughly checked.
There are a lot of ways to reduce & deaden ductwork & refrigerant line noise transmission. The noise issue can be resolved!