Switch it off when not in use! Seriously , a sound proof cabinet, put it in another place and pipe air where you want it
 
Put it in another room as has already been suggested. It needs to get plenty of air around it, not only to compress but also remember the pump and the electric motor is air-cooled. Making a sound-proof box is ok but might cause it to overheat, allow ventilation and the noise can escape - catch 22. Mine is in a small room off the main workshop.
 

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