Same problem an fresh install 16.04.
nano /etc/default/grub ----- GRUB_DEFAULT=0 #GRUB_HIDDEN_TIMEOUT=10 #GRUB_HIDDEN_TIMEOUT_QUIET=false GRUB_TIMEOUT=10 GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian` GRUB_CMDLINE_LINUX_DEFAULT="quiet splash" GRUB_CMDLINE_LINUX="" ... -----
Same problem an fresh install 16.04.
nano /etc/default/grub TIMEOUT= 10 TIMEOUT_ QUIET=false R=`lsb_ release -i -s 2> /dev/null || echo Debian` LINUX_DEFAULT= "quiet splash" LINUX=" "
-----
GRUB_DEFAULT=0
#GRUB_HIDDEN_
#GRUB_HIDDEN_
GRUB_TIMEOUT=10
GRUB_DISTRIBUTO
GRUB_CMDLINE_
GRUB_CMDLINE_
...
-----