Servers and NAS, by their very design, are not meant to be overly quiet. Honestly the xServe line is one of the quieter heavy duty server lines you will find. Average server room noise level is 95-105 dBA, between A/C and equiptment.
xServe is not meant to be a workstation so Apple did not go overboard making it quieter and it is doubtful anyone makes a kit to make it quieter.