Murph, all air conditioning units have two fans. One is inside the house in the air handler (also referred to as the blower coil, which may be mounted in the attic, a closet, or crawl space under the house) and pulls indoor air through the return-air grill (and filter) and blows it through the evaporator (cooling coil) and out the ductwork and registers. The other fan is part of the compressor unit outdoors and circulates outside air through the condensor coil (which ejects heat outdoors). The indoor/airhandler/evaporator fan should run 1) when the Fan switch on the thermostat is set to AUTO and the system/compressor cycles on to cool the house; and 2) any time the Fan switch is set to ON even if the compressor is not running. The outdoor fan on the compressor unit is not controlled by the Fan switch. It turns on only when the compressor is running. If, for example, the temp inside the house is 72F and the thermostat is set at 80F, then the compressor is not going to run. Setting the Fan switch to ON in such a circumstance will run the indoor fan for circulating air through the ducts, but the outside fan and compressor will remain off. Setting the FAN switch to ON will (or should) also circulate indoor air if the System switch (Off/Cool/Heat) is set to Off ... in which case the compressor will not run regardless of how the thermostat temp is set.

2 separate units will do the job, but cost of maintenance and break downs of 2 units as opposed to one with zone controls will in the long run be more costly. You have 2 units to maintain, you have 2 units to repair when they break. Tag team event. One system breaks down one month. The other system breaks down the next month. Doesn't always happen this way... but it could. zoning has to be done right. So what does that cost? I don't know I am not there. 'Design, Design, Design' other wise 'Trouble, Trouble, Trouble'.

I usually think that kitchen hums are from refrigerators -- maybe the compressor on yours has gotten tired and thus louder than it used to be? (and yes, a fridge turns on and off when running, just like your furnace does.)

Common Causes of a Constantly Running System. https://www.air-conditioning-ductless.com/ Dirty evaporator coil. Did you forget to have your air conditioner serviced this spring before you turned it on for the summer? If so, the evaporator coil may be covered with dirt and debris, impeding the cooling process and making the unit work harder to cool your space. This can happen even if it was cleaned a few months ago, and the unit is in a location where it picks up a lot of grime, like near a restaurant kitchen. Frozen evaporator coil. If your unit is blowing warm air from the supply vents in addition to running constantly, it may have frozen up. Turn the air conditioner completely off, and call in a professional right away to diagnose the cause and prevent further damage to the system. Clogged air filter. When your system’s air filter is full of dirt and debris, the flow of air is restricted, which can impede the functioning of your air conditioner. Without enough air flowing over the coils, the system can’t remove humidity and cool the air to the set temperature, so it keeps running when it shouldn’t. Restrictive filters. Even if your air filter is new, you may be using one that’s too restrictive for your system (such as one designed to remove pollen and allergens). For better air flow, use a filter with a lower MERV value. Low refrigerant charge. If you don’t have enough refrigerant in the system, probably due to a leak in the coils, your unit can’t cool the space. When this happens, the unit will keep running as it works harder. In this case, you need an experienced HVAC service professional to find and repair the leak and restore the refrigerant to the correct level. Faulty thermostat. If your thermostat is not working correctly, the unit doesn’t know to turn off even when the temperature is reached. Check your thermostat to see if the actual temperature in the space is below the set point. If it is, there’s a good chance that the thermostat may need replacing. Leaky ducts. Especially if you have older ductwork in your building, you may be losing cool air through leaks in the ducts. The joints may not be sealed, or the insulation may be gone. If enough cool air escapes, the temperature in your space never reaches the set point and the system will keep running. Dirty or blocked condenser. Just like the evaporator coils on the inside, the condenser coils on your outdoor unit also need to be cleaned, possibly even more so since they are exposed to the elements. The flow of air can be impeded by dirt and debris from the air, but also from accumulated leaves or even animal or insect nests. A good cleaning using professional products and tools will take care of this problem. Blower motor and fan issues. If your blower motor is not producing enough air for your system, or the fan is running at a low speed, that can cause the system to lose enough efficiency to make it run too much. Adjusting the fan speed can solve the problem. A layer of grime on the fan blades can also slow it down, so again you may just need a good cleaning. Incorrectly sized unit. Have you recently renovated your space or rearranged the layout without changing your air conditioner? It’s possible that your unit is no longer adequate for the needs of your space. Bring in a certified HVAC professional to do a load calculation, taking into account the design, size and needs of your building as well as windows and sun exposure, insulation and more. Many of these issues can be taken care of, and even prevented entirely, with regular maintenance of your equipment. What’s more, neglecting this task can be costing you in many ways that you haven’t considered. To learn more, download a copy of our helpful guide to Calculating the Hidden Costs of Poor HVAC Maintenance.