I don't know but my outside unit has been making a relatively loud continuous buzz when it is running. This has been the case for at least a couple months (that I have noticed) but the unit seems to otherwise be functioning fine (house still cools down well enough). My concern is that it could be an indication of a problem that should be tended to before a breakdown or other damage occurs.
needs oil..
needs to be leveled..
cicadas
Have you checked the breaker outside by the unit
The outdoor portion of the central ac unit is what gives up the heat from indoors, so it has to be outside. For a more thorough explanation see the question: How does air conditioning work.
keep it closed the windows .but if need fresh air from outside , open the windows.
No, but it's a good idea to protect any metal equipment from extreme weather.
What about them?
yes you can
Central air conditioning systems are designed to provide ventilation by bringing in outside air. Leaving a window open would tend to defeat the cooling effect.
If you are talking about elements, they are electrons.
When air is cooled the water in it comes out (like mist forming on the outside of a class filled with cold coke) and this happens in the air conditioner. Normally the dripping water is drained way to the outside (or a drain) by a pipe. If the pipe gets blocked the water will overflow and the conditioner will leak water - get it serviced.