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Your question does not include what type of system you are asking about, but the assumption is that it is a heat pump. The loud humming is the compressor running by itself while the fan is off.
This is the typical way a reasonably new heat pump operates:
There is a coil temperature sensor that initiates a defrost period to eliminate the frost/ice build up on the outdoor coil. In the defrost mode, the outdoor fan shuts off and the reversing valve in the heat pump reverses the direction of refrigerant flow to warm the outdoor coil. The same coil temperature sensor senses the temperature of the external surface of the coil and tells the defrost control when the frost is gone initiating another 'reverse cycle' to put it back into heating mode. In the heating mode, the outdoor fan runs again.
If you have the heat pump installed where snow drifts accumulate, you will have a problem extracting heat from the outdoor air, and coil frosting and freezing will be exaggerated. If you have excessive ice build up due to a defosting issue, the ice can actually stop the fan from rotating, but you would hear loud bangind and rattling for hours first as the fan blades come in contact with the ice.
If you do not have drifts, but continually have an excessive amount of ice build up, either the sensor is possibly faulty or the defrost control board is faulty. The sensor's clip could easily have been knocked loose from the coil by ice and is no longer sensing coil temperature and if so the result would be the lack of defrost initiation.
remove cover at ceiling so you can see in at fins and see if they are frosting up bad. If they are, I suspect during AC installation, they didn't open up ducts on discharge properly and air flow is restricted, which will cause freeze up, especially if low flow from discharge ducts. I'm assuming it is a ducted system. You may have to remove or drop down entire ceiling assy, to see if the ducts into ceiling have any blockage. Sometimes they don't cut rubber roof material back far enough, or misaligned ducts and cause restricted flow to ceiling vents, which will cause unit to frost up, which kicks out compressor until frost is melted, then compressor kicks back in and cooling happens again.
It sound like you have a direct short in the compressor ,Usually it means you have a bad compressor,but sometimes it could be a loose wire or bad outdoor motor. Ive installed about 10 of these units and still have yet to have a bad compressor.It could also be a bad circuit board in the outdoor unit.I wish I could narrow it down better but thats my best guess!Thanks ---Nate
Sounds like your compressor is seized or a start fault. If the unit is under five years old it should be under warranty. Call Fujitsu warranty out to have a look at it for you. Contact details as follows http://www.fujitsugeneral.com.au/contact.php
Sorry but either way I think your going to need a part there is very little that I can do unless your in WA.