Hi BB,have you definitely identified that it is coming from the PSU?
Can you feel any vibration in part of the case as it may be something thats slackened off a little and being allowed to resonate.
It may just be a noisy transformer,I have had some doorbell transformers that were quiet for a week or so then developed a buzzing noise as the bits between the plates seem to dry out and slacken off a litle allowing them to vibrate.Doesnt do any harm just annoying when its a constant buzzing noise