IF....you have an AIrPort Extreme, it is the motor bearings on the internal fan that are making the noise.
IF....you have an AirPort Time Capsule, it is either the motor bearings on the internal fan or the internal hard drive that are making the noise.
In either case, if the AirPort is covered under the 1 year warranty, take the product into an Apple Store.....if you have one near you....or contact Apple Support directly by clicking the link at the upper right of this web page.
The AirPort will also likely be replaced if you have AppleCare in effect on one of your Macs......AND....the AirPort is less that three years old.
Apple does not offer repairs or repair parts on the products. If the AirPort is covered under warranty, Apple will replace the product with a refurbished unit.
If the product is not covered by warranty, about all that you can do is open up the product and very carefully pull out the components. It might be possible to order a replacement fan for the product or a hard drive. Unfortunately, if you take the product to a repair shop, the repairs will likely be more expensive than buying a new AirPort.