Performed heating maintenance on the single system. Replaced both of the filters 1414 by one and 20 x 25 x 1. Inspected electrical connections and components. Refrigerant levels capacitor on mechanical items. Appear to be in good working order at this time.. As for the sound that is being heard, that is the defrost cycle and at times, especially when it’s colder, the defrost cycle will be louder and last longer. Technician tested the defrost cycle today and the defrost is normal, however, easy for someone to hear that and think possibly something need to be looked at or checked. To the service technician, the defrost tone is normal and the timing of the defrost is appropriate for today’s test conditions.