On Nov 24,  9:08, Dwight K. Elvey wrote:
  Hi
  Pairs of switching power supplies often interreact to
 make a sound like hissing. There may not be anything
 wrong with it. Still the electrolytic leaking might
 be a problem. 
OfficeConnect hubs, switches, whatnots, are very small devices that
have an external wall-wart PSU and no fan, and are normally absolutely
silent.
--
Pete                                            Peter Turnbull
                                                Network Manager
                                                University of York