I had been struggling with CPU fan being too noisy on my Macbook Air especially with an external thunderbolt display. This was the case with both Mavericks and Yosemite. I finally stumbled upon this article which helped me kinda solve the problem. the CPU usage is now normal 95% of the times and no more nosy fan. Please refer to the original article on apple forums by Luis_Mercado
Summary of the Fix is below (Please make sure you print out this guide and follow the steps exactly in the sequence mentioned)
First Step disconnect any external or secondary monitors, if any is present. The video memory allocation leak can also happen if you have a system with an integrated card, like Intel, with no external monitors attached.
Second Step Shut down your machine and enter Safe Mode (press shift once you turn on your computer again, more info below). Once you're there, fix your disk permissions.
Guide of how to access Safe Mode: OS X: What is Safe Boot, Safe Mode?
Third Step From Safe Mode turn off your machine again and reset your System Management Controller (SMC). There are different methods, depending on machine, to do this. To know what method applies to yours read the following guide.
Intel-based Macs: Resetting the System Management Controller (SMC)
Fourth Step Once your machine completes a full boot after resetting the SMC turn it off again and reset your PRAM (THIS IS THE MOST IMPORTANT STEP, BUT THE PREVIOUS ONES ARE ESSENTIAL FOR THIS ONE TO WORK). The PRAM stores small bits of data that indicate our Apple computer how to interact with the devices connected to it, including monitors and video cards. It also affects software.
To reset it you should hold the OPTION, COMMAND, P and R keys in your keyboard immediately after turning on your machine again.You'll hear the start up chime, continue pressing the keys until the machine boots and the chime starts A SECOND TIME, then release. IF YOUR YOSEMITE INSTALLATION LOCKS UP AT A BLACK SCREEN AFTER THIS, DONT PANIC! It's normal, just turn off your computer and let it boot again.
More info about PRAM: OS X Mavericks: Reset your computer’s PRAM