May I suggest that you consider having an electrician install at your main electrical panel/entrance a 'whole house surge and lightning protection' box to protect everything connected to your panel from such events? You may also have in addition to your HP & fan coil other sensitive electronically controlled 220V equipment such as clothes dryers, stoves, pool equipment etc. that would benefit from such protection. SR

It's most likly to be restricted airflow. Cause can range from dirty filter,dirty evap coil,collapsed return to blower not moving suffecent air. Did ambient outdoor tempature in your location drop recently? While waiting for tech to return I suggest raising temp setting to highest point at t-stat and setting fan to ON/CONT. After 6 or more hours set temp for normal operation but leave fan set ON/CONT. Carfully test volume of air flow at the start and each hour or two.

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.

(Sorry for the confusion about which post to reply to - I'm not familiar with forum ettiquet and had started a new thread when I should have stuck with this one). The 24anb6 is a single stage. All conversations are centered around a 3 ton unit. The ducts are in the attic space. The original house was 1140q ft. The addition is about 488q ft. The original house (1950s) has a metal ducting trunkline system that runs in a dropped ceiling in the 12' long central hallway. The new ducting is in a jumble in the attic of the addition, with a big box thing (a baffle?) and 4.5" ducts curling around everywhere - even back to the front rooms of the original house. Apparently code has changed and new 'jumper' vents needed to be installed in all the rooms? Before that ceiling was closed up, I got a glimpse up there and noticed that it looked like and orgy of Robby the Robots - probably not the most efficient thing ever. If it's true that with the ductwork currently installed would not support a more efficient unit in the future without ripping out walls, I think this makes an even stronger case for upgrading the equipment now. (I work at home and I plan on living at this house a long, long time.). I was told when to pick out plumbing fixtures, hardware, lighting, etc...I guess expected to be told when to decide on the other choices involved. If the GC had asked, I could be looking at paying 10 or 11K for exactly what I want - a 2-stage, high-efficiency, quiet unit. Instead I'm having to choose between paying nearly 9k for stuff I don't want or - it seems from this email - paying twice for the install and associated repairs in addition to the upgrade. I'm going to push for exactly what I want. The GC is working up estimates for the Carrier Infinity 17, 19, and 21 SEER units. I have no problem paying the difference in the cost of the equipment, or even the cost the have the install done 'right', which I assume means moving the trunkline up to the attic and having a single trunkline.* I'm sure things will get sticky when we start discussing who pays for what. It would be so helpful to be able to get objective estimates from a neutral party on what the original install should have cost and what the install that I'm requesting will cost. Is that ever done? I'll post any progress as it happens. Thank so much for your feedback. It has so helpful in getting clarity on this situation! ************ *The email I received this morning regarding my request to move forward with the estimates for the 17, 19 and 21 seer units: "The 17,19,21 Seer units besides being even more expensive will likely require more duct replacement, more sheetrock removal, etc. as this will snowball the work; time frame would increase accordingly. " and "Regarding the duct work replacement, it should be noted that the metal ducting trunkline system & some ducting that pre-existed in your home is still in place & that airflow supply system may need to be upsized as well, as the Seer rating increases, expanding the scope of other sheetrock & installation repairs as well." It would seem that he really doesn't want me to upgrade...From what I can gather the difference in cost of the equipment between the builder-grade Comfort series and the Infinity series is about $1,500 - about 1% of the total project cost.