https://wiki.archlinux.org/index.php/Dell_XPS_13_(9360)
Continuous hissing sound with headphones
Open alsamixer and set "Headphone Mic Boost" gain to 10 dB (See discussion on reddit). Note that this does reduce the volume slightly.
You may also run the equivalent command:
$ amixer -c PCH cset 'name=Headphone Mic Boost Volume' 1