could be some cable that fit the fan, could be dust, could be the lubricating film on the PSU's cooling fan bearing has evaporated, could be a faulty psu, it could be a lot of things, it's hard to tell
what brand is your psu? if the brand is bad it could be a bad psu
also seeing your pc is only 3months old you are still under warranty, if you are worried you can RMA the psu and get a better