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Buy an air filter for your fan. Clean fan blades of accumulated dirt regularly. Inspect the box fan to make sure that there are no loose external parts. Disassemble the box fan to see if there are any loose internal fasteners if the problem persists. Quiet any rattling the box fan might make.
You can help prevent a buildup of dust and grime on your fan by attaching an air filter to it. Look for a filter that is designed to fit the size and shape of your box fan online or in a home supply store. Make sure to attach the filter to the rear part of your fan, where the air is drawn in. If you like, you can also construct your own DIY filter and attach it to the front of your fan, allowing it to do double-duty as an air purifier. For example, if you have a 20in (51cm) box fan, purchase a 20in (51cm) by 20in (51cm) MERV 11 furnace filter and attach it to the front of your fan with some heavy-duty tape. Accrued dirt on fan blades cause bearings to become worn out, which creates a lot of noise from the fan. Every 2 weeks, you should vacuum the fan with the crevice-cleaning hose attachment. Wipe down the blades with a damp sponge twice a summer. In addition to causing damage to the fan, dust and dirt built up on the blades can blow into your home, reducing the air quality inside. Box fans do an amazing job keeping the house cool, but oftentimes they also cause a lot of noise. Look for looseness in the fan's housing and fan guards. Wedge a piece of cardboard between the edges of the blade guards if they are not firmly attached to the fan and their looseness is causing a rattle. Add a drop of silicone sealant to secure the decorative cap to the front guard of the fan if it is too noisy. Inspect blades to make sure there aren't any cracks, and replace them as necessary. Screw the spinner further into place and tighten it if it seems too loose. This will tighten the fan hub to the shaft and simultaneously secure the spinner into place. If you notice that the box fan makes noise on level or smooth surfaces instead of on cushioned surfaces, it might be an issue with its base. Pads might be missing, so inspect the base of the fan and replace its pads as needed.