Unit runs constantly but only reaches 45psi. Compressor doesnt sound the same either. All lines and couplers seem ok.

From the sound of it you have a problem with your thermostat when the temperature reaches to a certain degree the compressor should shut off automatically, if not it will run continuously until the evaporator freezes up. Do not try to repair it yourself have a certified technician a qualified service man to take care of the problem.

What temperature is this heat pump being run at? If it is low ambient temp outside this may be running in what is called defrost mode to where the compressor will run without a call for heat. This seems normal to me as long as the setpoint temperature is being reached.

Try cleaning the unit out and any coils you can reach (typically located near the bottom of the unit behind the toe grille). Do you hear the compressor running? The compressor is constantly meant to go on and off frequently to keep unit at a constant temperature. If you do not, the unit could possibly have a coolant leak or an issue with the internal sealed system including a faulty compressor, evaporate fan or main control board. Contact a professional move forward accurately.

For manualsonline.com

Have someone check your pressures & temperatures in your refrigeration system. Also check the compressor amps with an analog meter to see if the amps fluctuating. There might be something wrong with the compressor.

There HAVE to be a thermostat to cycle unit .Is there a sight glass on the liquid line .If there is normally you charge it up until you see no more bubbles in the sight glass.
There is never a constant pressure reading because pressure depend on outside temperature .
Thermostat for freezer will not look like regular thermostat.
I will be in the evaporator section

Small refrigeration units like this one like to fail to cool by developing a small leak and slowly
leaking refrigerant.
THERMOSTAT FAILURE CHECK:
A technician will check the thermostat using using a DMM (digital multimeter). But you can suspect the thermostat if you set the thermostat high and the compressor motor does not run,
then set thermostat low, and the compressor motor also doesn't run - in other words, the thermostat is failing to start the refrigerator's refrigeration system no matter what its setting - an obvious failure of the thermostat.
COMPRESSOR FAILURE CHECK:
If the compressor is dying, you would hear the compressor running almost constantly, but not
sounding quite right (uneven noise, clunking, tapping).
REFRIGERANT LEAK CHECK:
If you unit is low on refrigerant (due to a leak), the compressor motor will be running
almost constantly, but it will sound Ok.

There are a few things that happen most often on these vehicles. Air suspension compressor failure,A/S relay and a small coupler under the master cyl breaks. Listen to see if the compressor runs if it does check the air line coupler under the M/C to see if its broken,if not it,its possible the compressor motor is ok but the cylinder is worn and not producing pressure,you can check this by removing the air line on the compressor and checking the airflow out of the line(it should be significant). If it is it could have a leaking air bag also

I have worked on many makita compressors and have run-across this problem on a few of them. Your compressor has a check valve that is supposed to hold tank pressure after the switch reaches set pressure. There is an unloader valve built into the pressure switch that releases the pressure in the discharge hose after the unit shuts off. The unloader allows the compressor to restart with-out any pressure in the discharge line. If the brass check valve is leaking pressure back into the system, air will leak out of the unloader valve. Follow the discharge line to the tank and you will see the brass check valve. To test valve for fault, run the compressor till it builds around 80 psi and unplug the unit from the wall outlet. Remove the 1/4" hose connector from the check valve and check for air leaking out of the valve. If it is leaking you can order form www.toolpartsdirect.com. or call the makita 1-800 number. The price is about the same. Good Luck on your repair.

What is outside (ambient) temperature. When was entire unit last cleaned? (evaporator coils, & condensor coils)

The only thing I can think of that you may be hearing would be if the compressor has a problem. If you notice cooling performance decrease or it stops cooling, I believe you will find the compressor has bad internal valves. It will probably take a technician to properly diagnose this and if the compressor has this problem the only answer is to replace the compressor.
I hope this helps, supertechks