Take the side panel off. Roll up a piece of thick paper or cardboard, or use a long funnel if you have one, and put the small end in your ear and use the large end to isolate where the sound is coming from. You'll need to identify WHERE, EXACTLY, the sound is coming from before anybody can make any recommendations on getting rid of it. You may even be able to identify where it is coming from by simply taking the side panel off and simply listening, with your head close to the case.
In other cases you might need to use something like a mechanics stethescope or a long screwdriver, which you can use to put up against the frame of each fan, or the graphics card, or the CPU cooler, or the PSU, to determine which of them is causing the noise. There are no moving parts in any PC except for the fans, so it MUST be a fan causing the noise. Whether it's because of a faulty fan or something loose/not secured, you'll have to determine once you figure out WHAT is making the noise.