I am a hvac service Tech and Wild Bill gave you sound advice. I suspect it it defrost control board or one of the sensors. It is not going into defrost or the reversing valve is not shifting into defrost. If the outdoor fan was bad it will ice up quickly but the defrost board will send it into defrost, it will defrost and start the cycle all over again. It will however cost you a lot of energy to operate it like that. The noise is the outdoor fan blade striking ice build up and possibly destroying it. Turn off disconnect or turn system switch on thermostat to emergency heat to prevent compressor operation until you can have a service tech diagnose root problem.
If you caught it early enough it should not of cost you too much. I don’t live in your area, to be able to assist you. Good luck.
Peddler,
make sure all the drain holes at the bottom of your heat pump are clear. I would look at the freeze pattern after a defrost cycle. It should be even on all circuits. There are generally 2-4 circuits on the outdoor coil. Each coil should look the same. It looks like your unit is going into defrost but the condensate is not leaving the outdoor coil area (water pooling up) When unit comes out of defrost the lower coil refreezes water. It is very tough to diagnose the root cause without putting the unit through it’s different sequences of operation. Check the basics like clean coils, clean filters, clean condensate pans and verify there are no excessive vibrations and tight electrical connections. These are things most guys can do with basic hand tools and knowledge to make their systems live a long trouble free life of service. Outside of that you need to find a reliable and trusted service tech.
G-spot is correct, if your unit is 15-20 years old it is time to start budgeting for a new unit.The old reliable refrigerant R-22 is not being made any more and the cost of it continues to rise as it is depleted world wide. HVAC systems are engineered around the refrigerant. It is not a good idea to retrofit a system engineered tor one refrigerant to another except as a last case senereo. Other than the refrigerant, almost all other parts are available to repair a unit including coils. If your unit is 15-20 years old and you have a major repair like replacing a coil or a compressor you will be money ahead to replace the unit. The new unit will have an up to date refrigerant and higher efficiency ratings but generally it requires new outdoor, and indoor unit as well as new line set. The ductwork can be reused.