Why is my heat pump making a temporary rushing water/ crackling sound/ vibrating noise?

All heat pumps will have a defrost cycle that will take place more often as the temperature drops. After running for a period of time, the system creates frost that must be melted away from the outdoor unit. This happens by temporarily reversing the flow of refrigerant which makes a strange sound in the home. This can be as often as every 45 minutes or sooner in colder temperatures; it is necessary for your system to maintain its efficiency.