When your air conditioner is working properly, it should make very little noise. There will be the obvious blowing noise when it's turned on, and depending on the model, it may make a clicking sound when it first switches on or off. But a rattling sound is a sign that something is wrong. The good news is that rattling is usually cased by a minor issue that you can solve yourself. Here's a closer look.
Sometimes the cage around your outdoor unit or the cabinet around the indoor portion of your air conditioner may work itself loose. It will then rattle back and forth whenever the fan is running. Go outside and check the condenser cage to see if it moves from side to side when you push on it. If it does, locate the screws that hold it onto the unit, and tighten them. If a screw is missing, just replace it. Inside, look at the AC cabinet, and make sure the cover is fully locked into place. Sometimes, the metal tabs that lock the cabinet into place become loose and don't work well. If this is the case, a little piece of duct tape will hold the cover in place and stop the rattling.
If little stones, twigs, leaves, or other debris have made their way into the outdoor condenser unit, this may be causing the rattling. Some of the items may even get sucked inside, where they then rattle inside the indoor unit. Turn the power off to both your indoor and outdoor units, and then use a shop vac to clean them out, removing any debris.
Once your AC system is debris-free, take steps to prevent additional debris from making its way into the unit. You may need to put a fence around your outdoor unit to keep debris from blowing in, rake more often so leaves don't end up in the unit, or trim a nearby tree that's been shedding twigs into the AC condenser.
If securing the cage and removing debris does not get rid of the rattling noise, then the issue is likely in the motor. A bolt may be loose, or a ball bearing may be wearing out. Unfortunately, this is not an issue you should attempt to repair on your own, as doing so may void your warranty. If this is the case, you should contact an HVAC technician like those found at One Hour Air Conditioning & Heating (West Pasco) to schedule a repair appointment.