We got a very dusty 27-inch iMac from 2010. Due to overheating, the Power Supply Unit was damaged and the HDD was slightly damaged. Completely clogged air vents caused the temperature inside the iMac to rise considerably and corrupted the computer. If you hear that your iMac fan is way too loud, even when it’s not under heavy load, your iMac may need cleaning. Don’t wait until it’s too late.