With fans running, put your finger on each fan to see if it changes the sound quality that you're hearing. If you have rubber fasteners that decouple the fans from the case, it's probably not those fans. If you have ones that hold the fans directly on the case then that's probably the culprit. Look into different mounts or silicone gaskets.
If the noise is consistent and it's not your case fans, it's probably your PSU. They make gaskets for those as well.
-z