The iMac contains an intelligent control system that monitors computer temperatures and delivers the right amount of airflow to critical components using three quiet fans. During CPU intensive activities the fans speed up to provide additional airflow. In a quiet environment the increased airflow may produce an audible noise that sounds like rushing air. If you hear unusual fan noises, or if the fans seem to run at full speed even when the computer is idle, try resetting the SMC.
The optical drive spins a CD or DVD at a high rate of speed during use. If your optical drive vibrates or makes a lot of noise when you use it, there may be an issue with the disc in it. Try using other discs to see if the issue is isolated to one disc. The disc’s weight may be slightly off-center because it is uneven or because of the ink on the label. If there are labels or stickers on the disc try removing them. This imbalance won’t damage the disc or your computer.
Hard drives generate audible noises when performing read/write activities. Even when a computer is idle it may be performing background tasks such as Time Machine backups or Spotlight searches that utilize the hard drive. These noises may be described as grumbling, muffled thudding, or ticking in short bursts, or as a quiet tapping noise like raindrops. These are all normal operating noises and do not indicate an issue. You can verify the health of your hard drive with the Disk Utility application.