If it still runs fine and meets your needs, why not build a sound barrier around it ? Thin simple walls covered with egg cartons works. The shape of the egg carton breaks up the sound rings/waves.
I agree with this ^ There are lots of ideas on the web, such as: http://www.cianperez.com/Photo/Expo...ssor_Enclosure/Pages/Compressor_Enclosure.htm which can give you a pretty good idea on how to build one. I got the idea for the sound deadening box from folks that have noisy generators. Same principal there as well. The box, if built right, will deaden the sound considerably.
