You may be hearing the head pressure equalizing in the compressor, after it kicks off on overload. The Reversing Control Valve may be stuck. It could be a number of things.
Some units are not made to run below freezing, not sure about that one. I did not find much info on the model number that you provided. Below 32F and efficient drops way down.
Is your Aux heating strips working ?
It may be best to call the Man.
Good Luck.
Is this what you mean about it (the fan?) not running below freezing?
someone showed me the installation instruction manual here
http://iompics.com/downloads/Heat P...e/Install Instructions 25HCC5/25hcb-c-5si.pdf
and I saw this:
Defrost
The defrost control is a time/temperature control which has field
selectable settings of 30, 60, 90, or 120 minutes, factory set to 90
minutes. These settings represent the amount of time that must pass
after closure of the defrost thermostat before the defrost sequence
begins.
The defrost thermostat senses coil temperature throughout the
heating cycle. When the coil temperature reaches the defrost
thermostat setting of approximately 32_F (0_C), it will close,
which energizes the DFT terminal and begins the defrost timing
sequence. When the DFT has been energized for the selected time,
the defrost cycle begins. Defrost cycle is terminated when defrost
thermostat opens, or automatically after 10 minutes.