An air conditioner should circulate chilled air in the room without really announcing it. When your AC sounds like a jet airliner taking off just above your roof, it kind of ruins the restfulness of the situation. There's undoubtedly a reason for this abnormally loud noise (which is probably growing louder by the day). Some of these reasons may not be repairable.
A failing air conditioner compressor (located in the outdoor section of a split system AC) will be loud, producing a roaring, strained noise. All AC units produce some operational noises, yet a failing compressor will sound like the volume has been turned up to near-intolerable levels. If it's noticeable inside the house (with the sound emanating from the unit's wall-mounted evaporator), go outside and stand next to the unit's outdoor component. The noise will be worryingly loud.
A compressor on the verge of failure (likely to be the case in this situation) will not only be loud but also inefficient. It will fail to freely circulate freon (refrigerant gas), meaning that the air expelled from the indoor evaporator will be noticeably warmer than it should be, and possibly humid—regardless of the thermostat setting.
Fan Belt and Motor
As the unit's outdoor fan attempts to compensate, it will become strained. This can add even more noise, as the fan belt and motor may rotate with added intensity. This will create a rattling noise, or even a grinding noise if the fan belt should loosen (which may be a possibility if the problem has been ongoing). In extreme cases, the friction created by a loose fan belt may start smoking, which you can smell as it mixes with air being expelled from the indoor evaporator. Instead of straining an air conditioner that has reached the end of its service life, you should deactivate it. This is critical if you should smell smoke.
The unit will have to be professionally inspected. It may be possible to replace faulty parts. However, your unit could be an older model—which may well be the case, since a failing compressor and faulty fan and fan belt can be due to wear and tear associated with standard use spread across many years. AC replacement may prove to be a more cost-effective option.
An intolerably loud AC points to a failing compressor, and in some cases, a failing compressor indicates that the entire air conditioner has come to the end of its service life.
For more info about AC replacement services, contact a local company.