I had power supplies that whine i am not sure what causes it
When capacitors go bad they'll start to make a screaming noise. Which is bad. Transformers can cause more of a buzzing noise which is fine. Without to long of an electronics lesson the magnetism switches polarity in the transformer to cause it. Usually you need sharp ears to hear it (I hear my PSU buzz, but I can't hear it over the lights when they're on if you can believe that).
If the sounds always been there its usually fine, it's when it's a new sound that you should check it out more.