I noticed the fan was constantly making a lot of racket in my MacBook Air lately (running Yosemite 10.10). VirtualBox appears to be the culprit, even when my Windows 7 guest is idling (showing as 100%+ in Activity Monitor).
After reducing the number of assigned processors to 2 (was previously 4) everything seems to be back to normal. I also set the max usage to 75% (although this had no impact when set to 4 processors, so i’m not sure if it actually helps).
Looking forward to the increased battery life more than anything.