usually it's inductors that squeal. often referred to as "coil whine". solid caps are highly unlikely to vibrate. between a number of the x-XXX gold rated units, i had never gotten an audible coil whine. however, recently bought and returned a 660xp2 platinum unit that had pretty bad whine. could be a change in component sourcing, contract manufacturer, or something inherent to the design. i will say that seasonic _always_ uses enclosed inductors (encapsulated with epoxy and then sealed into a metal box) when possible. there's also a possibility that it is a weird high order electrical interaction with your particular power drains and the supply.
i will say that switching power supplies have to be switched at extremely high frequencies to get this kind of efficiency, so it's not that surprising to get a little bit of coil whine (or a lot in a budget efficient unit). seasonic is one of the best OEMs, however, carries the best warranty, etc.
