You must have become accustomed with normal sounds that the air conditioning unit generally makes. However, when it begins to make unfamiliar noises, there is reason to worry. Excessive air conditioner noises should not be ignored as these might point towards some serious snag. The air conditioning system's sound can be used to diagnose the nature of the problem. Ignoring it might worsen the situation to the extent that repair becomes difficult and expensive.

Continue reading to know some of the common noise sources that might disturb the window unit and what can be done to prevent these.

Fixes for Noisy Fans

For a fan to work properly, it should be kept clean and well maintained. The first step is to find the source of the noise. Fans produce a rattling or harsh humming sound due to loose-fitting or inadequate cleaning. Remove any debris you see on or around the blades. You can even check to see whether the blade itself is loose and get it tightened if required. Also, make sure that the blades are fitted straight. A blade with even slightest bend can touch inside of the unit thereby causing harsh noise.

Lubrication

A motor that needs lubrication may emit a humming or grinding noise. Some units come with unsealed motors so that you can apply lubricants easily. Further, occurrence of noise can be prevented by oiling the air conditioner motor at the onset of each summer.

Loose Screws

When an air conditioner works, it vibrates automatically. This vibration can cause screws in the unit to come loose. These screws can be present in panels throughout the unit. Then there are mounting screws which hold the unit in place. Both situations will produce a rattling noise when the air conditioner is putting out air. These can be fixed easily with a few turns of your screwdriver.

Coil Fins

If the coil fins become clogged with dirt or bent, the air conditioning unit may become noisy. The debris can be cleaned using a vacum cleaner and bends can be removed using a special tool called a fin comb.
In order to nip the problem of a noisy air conditioner unit in the bud, you can even follow these simple steps:

Lower the heat load on an air conditioner by insulating the ceiling, and shading east and west-facing windows.

Check the label of sound power level before purchase, the smaller the dB(A), the quieter the unit.

Place the air conditioner on the rear wall facing the backyard, or on a front or side wall facing the street. Split system compressors should be mounted on rubber blocks to reduce vibration and can be located away from the wall.

Install an acoustic enclosure. Those which are custom-built with correctly designed ventilation serve the purpose more efficiently.

Ensure that the central air undergoes regular maintenance.

Handling a noisy air conditioner is typically a job best left to HVAC experts. So, if the problem persists or aggravates, a HVAC professional should be contacted immediately.

Reference:
http://atcair.com/what-to-do-when-your-air-conditioner-starts-to-make-strange-noises