I have a 4 ton Carrier Heat Pump Heating and AC unit. Lately, the unit has been making a horrendous, loud bass trombone sound. This sound comes from the outside unit that houses the fan and compressor. It occurs when the heater/blower in the atic is NOT on. The sound is so loud that it wakes me up at night, and my neighbors over 300 ft away have complained about the noise. The sound is unique and really undescribable. The people who installed it 3 yrs ago say it's normal when the unit literally freezes up (ice accumulates on the outside base of the unit. It has not been cold enough for that to happen here in the San Antonio, Texas area) but they have never personally been out here to hear it. They just don't want to be bothered, I think.
Anyone have any idea what is going on?

Well it sound to me when ur unit goes to defrost it will make a loud bass sound and that is from the reversing valve changing the direction of the freon it should only stay in defrost no more than ten minutes