The one vacuum hub on my Jimny was broken and it caused a scraping/grinding noise when driving on dirt roads. This video shows how to remove and replace the hubs. I also show what the hubs look like inside and how they work.
The vacuum hubs connect the front wheels to the drive train when in 4WD mode. In 2WD mode the front wheels can turn without turning the front drive train, which reduces wear and tear on the front diff and increases fuel efficiency.
