If your central air conditioning system's outdoor unit is making distracting or unusual sounds, such as rattling or buzzing, then it may be caused by one of several common problems. Fortunately, these problems aren't serious and can be corrected by most homeowners willing to spend a little time diagnosing and making repairs. Below are three possible causes for a noisy outdoor unit and what you can do to correct the trouble:
A metallic, rattling sound in your outdoor unit may be caused by a poorly-secured cabinet. Loose screws permit the thin, sheet-metal cabinet to vibrate and rub against surrounding metal parts. Since there are possibly dozens of screws used to fasten the cabinet, there is ample room for some to come loose over time.
To correct the trouble, find an appropriate-sized nut driver that matches the heads on the screws holding the cabinet in place. Next, begin tightening each screw in a systematic fashion around the exterior of the unit, so you can keep track of which ones you tighten. Most screws will probably not need any tightening, but if you find any, be sure to secure them. Placing a drop of thread-locking compound on the screws' threads will help the screws stay in place longer, but this is optional if you don't mind periodically checking and retightening the screws.
For screws that won't tighten, wrap the threads with a couple of turns of sewing thread and use a drop of thread-locking compound to hold everything together. This should help provide extra grip between the screws and cabinet.
Another common cause of noise in your air conditioner's outdoor unit is debris that becomes trapped in between the coils and cabinet or other components. Likely culprits include leaves, twigs, animal nesting materials, dirt and sand, and plant stems. In addition, other objects such as paper scraps or toys can find their way into the unit and create objectionable noises.
For some outdoor units, you can remove these objects by simply pulling them out of the gaps between the coils and cabinet or from the fan outlet at the top of the unit. However, you may need to access the interior of the unit to access other items that are deeply buried beneath the wires and parts.
In that circumstance, be sure to power off the unit first by flipping the main outdoor switch or by removing the power handle, depending on your unit's make and model. Next, remove the screws holding the cabinet in place and set them aside in a location where they won't be lost. Carefully pull the cabinet away from the unit, being careful not to pinch or sever wires.
Once you have access inside the unit, remove any debris you find. A shop vacuum is particularly helpful for objects you can't reach, though be careful not to dislodge any connections or damage the sensitive fins that line the coils.
Most outdoor units are situated on a hard pad that provides a firm, flat surface for operation. This is important to the well-functioning of the unit because it absorbs vibrations and provides a level base for the internal components. If it the unit isn't level to within a few degrees of vertical, then it places stress upon the moving components inside the compressor. This can lead to excess noise generation and even cause wear-and-tear beyond what would be expected.
To check if your unit is level, simply lay a box level across the top of it in at least two dimensions. If it isn't level, then you can insert wooden shims beneath the unit to make it level. Alternatively, you can also raise the pad itself using two-by-four boards and packing sand beneath to make it level. Just be sure to be cautious when handling the unit to avoid damaging the refrigerant line connections.