it may be the equalization of the freon. Since your system does not make the noise in the winter, and the reversing valve is probably energized in cooling, when the cooling cycle is satisfied the reversing valve may switch back causing this noise from the pressure equalizing out at a fast rate.

The gurgling and buzzing is normal and really exists in all units (i.e. it is likely you have nothing wrong but an irritating noise). I even listen for this noise when I first approach a unit because it gives me sign of it's "health" so to speak. There is an explanation and several solutions which use a lot of technical terms and even some elaborate devices which can be added to the system but, I won't go into all that right now. I'll explain as simply as I can and hope you understand (and also hope I explain it simple enough!!!)If the noise is noticable but not irritating or disturbing, I think the following is most likely the problem. It is associated with the refrigerant as it cycles through the line sets. If the lines touch the structure anywhere along the hollow wall, the noise can be transmitted more noticeably. It may be that where the lines enters the attic space, they are affixted to a joist or just laying against it. If they are affixed without a vibration dampening means, the noise will be more pronounced. A rubber grommet inside a fastening clamp usually minimizes the noise. The air handling unit where the evaporator coil is mounted should also be dampenedor the gurgling and buzzing will be heard also. Although there is not much noise associated with it in the way of gurgling, there is a condensate drain to drain off the evaporator coil. The buzzing is usually associated with the refrigerant leaving the metering device and the gurgling is associated with the liquid refrigerant going up the line to the metering device and some can be heard as the refrigerant is in the evaporator coil. When the unit shuts off, you may hear this noise more clearly for 30 seconds or so. I hope this helps you understand and if you should call a technician to remedy the problem, you'll have a better idea of what he'll be doing. Good luck and feel free to email me if you need more info!!

I've got this same problem. My hum/buzz sound starts off quiet then gradually gets louder and then fades to quiet. This repeats over and over. Goes on for about a good 40 minutes or so. ****Get this: As I was typing this, the noise started. I ran next door for my neighbor, he is an electrician. He traced the sound to outside...traced it all the way to the meter on the side of my house. The gas meter line. So I am calling the Gas Company tomorrow first thing. I have had this noise for a good month now. He lives on the other side of my duplex and he never hears it. I'll write back tomorrow if the Gas Co. says anything.

"Yes but is the explanation, "the wire vibrate in the conduit" reasonable. Can this even occur?" When the unit starts up the first surge of current is up to the "LRA: 68 amps". LRA is 'Locked Rotor Amperage." Since the motor is NOT turning yet, it pulls a large current surge for a few cycles of the 60 Hz power. This CAN cause wires in a metal conduit to move around. It is less common in non-metalic conduit. It is not a problem. As long as the unit starts up without tripping the breaker, and starts up quickly (no more than a second or two) it is fine. The current will drop to the "RLA: 14.1 " value or less so quickly the breaker does not trip.