It is probably the black plastic shroud around the fan making the noise. You can check it and make sure that it isn't too close to something or that the screws or pins that are holding it in place aren't loose. Also, I noticed that there was black tubing in your case. Is that a CPU liquid cooler? I just installed a Corsair H60 on my CPU yesterday and I noticed that there was a fairly audible buzzing noise. At first, I thought it was a fan or the case itself. But, I looked it up online and found out that these closed loop liquid coolers make a buzzing noise sometimes if the voltage to the pump is too high. If you do have a CLC and the pump is plugged into a motherboard header, check if it is a 3-pin or 4-pin header. The easiest way to find out if the pump is making the noise is to unplug it then turn the PC on for a few seconds. If the noise is gone then it is the pump. If it is, you can do a few things like rig the pump to a 7v or 5v source using a free molex plug, use a fan speed controller, put a resistor on the pump's wire (which will probably void the warranty) or try it in a 4-pin header.