Why is my outdoor air conditioner unit making a screeching/squeeling sound?
If your heating system is purely electrical (see comments), then your outside unit is not an Air Conditioner, but a Heat Pump, who's job is to take heat from outside (ha ha for you New Englanders and Mid Atlantic types) and bring it inside.

So your system is squealing because it's running as its supposed to, only it's cold outside, and the parts need lubrication.

Have your system checked/maintained by a qualified Heat Pump technician.