I have always loved my iMac because how quiet it is. However, a few weeks ago the fans became clearly audible. I installed smcFanControl to monitor the fan speeds and temperature. My iMac is running at 75C and the fans are spinning at 2800-3200 RPM at idle to light usage. We are talking about a 2 to 3 tabs in Chrome open. I made sure no other programs were running in the background, and confirmed in Activity Monitor that CPU usage was low (<5%). I am scratching my head to figure out what has changed. I did a fresh install of 10.10 to ensure there were no programs hiding in the background. I booted into Windows 7 to check to see if 10.10 itself was the culprit. In the former case my iMac continued to run at 75C and 2800 RPM. However, in Windows 7 it seems to run marginally cooler. The CPU is in the high 50s, the GPU is 55C and the HD is 42C. Additionally, I checked the ports for dust and the both appear clean. Later I will blow them out with compressed air. Windows is running almost 20C cooler than Yosemite. I've Googled and Googled, but I have yet to find anyone with the same problem (and better - a solution). I'm going to install 10.9 to see if it run any better than 10.10. If anyone else has experienced something similar, I would really love to hear what helped. I suspect some feature or design in 10.10 is causing my iMac to burn more energy but for some reason I do see it reflected in the Activity Monitor.