Every time after my Mac (MBP late 2013 with OS X 10.10.5) starts Screen Saver mode and then after some time turns off the screen (even if no program is running) the fan speed rises dramatically.
Is it normal, or does anyone know how to fix this?
Some native screen savers can cause this problem as well (don't ask me why).
Just pick a simple plain one and see if it still happens (try a few of them).
Hope it helps :)