If it still occurs, you have to find out which component makes the noise. The best is to start with the fans. While the PC is running, stop every one of them with your finger and listen if it was the fan you're holding. Don't worry, if you approach the fan slowly and you don't put your finger between the rotor blades. Be careful you don't touch a circuit board. If you still feel uncomfortable, just take something soft, like winter or leather gloves. It doesn't mind if you stop a fan for a few seconds, just watch it, if it starts spinning again. If it doesn't spin in around 3 seconds, give it a pull in the rotating direction.
Write it down here if you could find the noisemaker.