Central air conditioning is a wonderful thing, but the unit that sits outside your house (called the condenser) can be irritatingly noisy especially if it has been installed under a bedroom window or similarly unfortunate location. Even with the windows closed, it can get pretty noisy especially if you or your guests are trying to sleep.
To help quiet the beast, first make sure it has been properly serviced by a reputable HVAC service and repair techncian. Be sure to ask the HVAC technician to check for loose sheet metal and to check the fan blade clearance. Vibrating sheet metal is noisy and sometimes just securing the sheet metal housing will significantly reduce the amount of noise it is making.
Building an enclosure around the unit is a possibility, but be careful to allow for sufficient air flow. Ask your HVAC technician how much space to leave around the unit. Sometimes good results can be achieved by enclosing the top and sides closest to the house and leaving the side away from the house open or covered with lattice. Also, you will need to make sure that the enclosure allows for easy access to the entire unit for servicing.
Relocating the unit to a better location can be very effective. There is some cost involved because your HVAC contractor will need to run new coolant lines and do some electrical work, but sometimes just moving it a few feet can make a big difference.
Finally, when it comes time to replace your HVAC system, look for quiet running unit. Central air conditioners are rated for noise and some models are much quieter than others.
