Many of the users are reporting that upgrading to Mac OS X has caused their Macs to run hotter in general and due to which their fans remain constantly engage, creating excessive and unusual fan noise. In case your MacBook Pro fan is always awake then it may led to several other problems like invariable fan speed, or excessive noise. To fix this simply follow the steps:

First of all open the activity Monitor to see which applications are using the most CPU. By this, you can also check which apps you are using the most. Applications like Final Cut Pro, Photoshop, and VMware Fusion/Parallels tend to cause the fans to speed up, as the temperature of your CPU/GPU will boost up when using those applications.
Now disable flash in your browser. In the Chrome, you can use Flash control settings to disable instances of Flash. It is one of the biggest causes of CPU/GPU stress, and will no doubt amplify your MacBook Pro fan speed because Flash is so exhaustive on hardware.
After this restart your MacBook Pro and if your device fan is continually running at a high speed, try a Restart.
If a restart doesn’t work, then reset the SMC and PRAM settings. Or simply contact a third party support providing company that offers reliable customer service for MacBook pro to get rid of your problems easily.

Fixing a Noisy MacBook Pro Fan
If your MacBook Pro fan is still running fast and making so much noise, it may indicate its running faster than it should be, or there is something initiating it to do this.

Ensure that it is not clogged with dust. Fans are noisy when they have to run faster, and it may be running faster because it is clogged with dust particles. Dust particles will radically reduce the effectiveness of your MacBook Pro fan. It is moderately easy to clean and maintain the Apple MacBook Pro fan, without opening your MacBook Pro device. You can clean it by using a can of compressed air tool or dial toll-free Apple MacBook pro help support number and get the quality solutions from expert technicians immediately. Applecustomerservice.Us

Sometimes you may need to replace the fans if there is a severe damage in them. Old fans are normally louder than the new ones. In case your fans are always noisy from the start, then this would signify that the fans have simply worn out.