I'm running Yosemite PB4 on a 2012 MacBook Air 1.8 8GB RAM. I've noticed and it hasn't improved with subsequent betas that my fans are running much higher/louder than when I am booted into Mavericks. Mavericks, no load, no video fans run sub 2k, and you can't hear them. Yosemite, no load, no video fans run 3,800-4,500 on a consistent basis, and you can definitely hear them. In Mavericks I'd have to run video for at least 5-10 minutes to get above 4k fan speeds. The only thing I can think of other than a Yosemite issue, and I'm not sure what the correlation would be, is that Yosemite is loaded onto a partition on a Seagate 4TB external USB 3.0 drive, where as Mavericks of course runs off the internal drive. Should running the OS off an external cause this?