Responded to a report of a gurgling noise coming from the air exchange unit. Upon arrival, the system was turned on and found to be operating normally with no abnormal noise present. Carole and Mark reported that the system was turned off approximately two weeks ago when the noise first occurred and had not been operated again until this visit. Inspected the air exchange unit and all associated components. Confirmed proper operation of the blower fan, belt-driven enthalpy wheel, intake and discharge shutters, and all intake and discharge terminations. Inspected the ductwork and confirmed that all ducts are clear of obstructions with no visible signs of issues within the ducting. At this time, everything is operating properly with no identifiable cause found for the previously reported noise. Moving forward, I recommended that Carole and Mark document as much information as possible if the noise reoccurs, including outdoor temperature, current weather conditions, any snow or debris