More than likely a fan that is making the noise.
In my sys, all of the fans are attached to a manual fan controller, so i would just turn each one off, then back on to see if the noise stops.
Without a fan controller, you can still stop them manually be either unplugging the attached molex, or by stopping the fan for a short moment with your finger (be VERY careful using this method as it can hurt if you catch your finger in the blades, the best way is to gently press on the circular area in the centre of the fan to stop it).
If when you stop the fan, the noise stops, then you have found the source of your noise.
Once you have located your noisy fan, you should try cleaning it, as dusty blades can often unbalance the fan, causing the noise. To do this you can either just give it a damn good blow, or remove it (after powering down your system) and manually clean the blades. If it is still noisy after giving it a clean, then just buy a new one.
Possible fans causing the noise could be:
CPU heatsink fan
Northbridge chipset cooling fan (if you have one)
Graphics card fan (stock GFX fans are usually pretty noisy, although quiter alternatives are available)
Any other intake/exhaust fans on your case
Finally, if it is none of the other fans, then there is a posibility it could be your PSU fan(s), try giving them a quick blow.