I apologize in advance because I am a complete idiot when it comes to anything remotely mechanical... so bare with me. :)

I live in a building of condos where each unit has its own central heating/air unit. The unit is controlled by a single box on the wall, with one switch that controls heat/AC, one switch that controls the fan. It has a timer function and, basically, is all electronic. There is one main vent with ducts that run above my ceiling (I think, on the roof). I have no idea what brand name it is or the type of unit. My heat's working fine, and the filters were recently changed.

Anyway... it's cold here, and so I've had the heat on for most of the winter. I haven't ran my AC since September. I went out of town recently and had someone watch my condo for me. They basically had the heat on really high, so it was sweltering. I usually have the fan running, but that wasn't helping. I probably should have just opened all the windows and let the cool air in... but I didn't and just flipped the switch to turn on the AC for a minute or two.

I went into my kitchen and thought I heard someone banging quietly on the roof (I live on the top floor of my building). I didn't really think anything of it. I then heard the AC kick in. A few minutes later, it sounded like someone was running a garbage disposal--a loud humming sound--with vibrations. A lamp on a table actually moved. I realized it was coming from the AC unit, and when I flipped the switch back to heat, the noise and vibration stopped altogether.

After experimenting with it, it seems like the AC only makes the noise when I set the AC to a temperature that's below the current temperature in the room. If it's just set on AC, and the temperature is 73 inside and the AC is set for 73, it's fine. The AC seems to be working, as it cools down the room quickly. Everything else seems normal... just that awful vibration/noise.

I'm wondering if the problem is stemming from the cold weather outside... we had a freak snow storm recently. Would it be better to wait until it's warmer and then see how it acts--or should I call a professional to look at it now?

You may have had ice on the outside ( cond.) units fan blades, and / or the compressor was starting in cold weather without a crankcase heater, meaning all of the refigerant has migrated to the outside( colder temp) and was slugging the compressor with liquid refrigerant, a BAD thing) Starting the unit with ice on the blades may or may not have caused damage to the blades ( causing the vibration) If they are damaged, they (it is a one piece unit ) will have to be replaced. A HVAC tech will have to check and make repairs if need be...