You may also want to take a look at any lose wires in the case, sometimes when a free wire from a fan or power supply cable get too close to a fan they'll cause a very fast paced sound, almost like a hum or a buzz. Its kind of like a trading card in the spoke of a bicycle wheel. Poke around a bit and see if there are any cables close to fans in the case.... typically they'll be louder at startup and as the fans get faster or time goes by the cords will move away from the fans or simply by vibrations of you walking around the desk/case making them go away or lessen.
I used to have this problem on my case till I learned cable management
As for case vibration you could place the case on a flat surface (desk/floor) and leave it on letting it buzz... simply move around the case pressing in on the corners and pannels till you get the noise to stop. Once you find the culprit maybe just tighten up a lose screw, or try some foam or cork padding from your local office supply store.
And if it is the power supply, sadly there is just no way to know without getting close and listening where the noise is coming from and even then you'd have to replace it with another PSU just to know for sure.
Best of luck, let us know how it turns out