I set mine on "auto" in the winter and leave all my vents open...front and back. Does a nice job of getting the interior warm quickly. I agree it sounds like a leaf blower on the highest setting when you set the fan speed manually.
2015 BMW F25 35i M-Sport
Alpine White / M-Sport / Mocha Nevada /Fineline Anthracite / DHP / Adaptive LED / HK Sound / 20" 310M Run-Flats