Check which folder is using all the disk space

du -h /folder | grep '[0-9\.]\+G'

 You could also use /, but might be slow on large hds. I had disk filling up slowly but surely and found that the cause was log file on docker container:

23G     /var/lib/docker/containers/d8axxx
23G     /var/lib/docker/containers
24G     /var/lib/docker
24G     /var/lib
25G     /var
26G     /