I recently began experiencing my system fans kicking in and running all the time. The fans began running so much that I almost didn’t notice them any more. When I pulled the Activity Monitor up, the md worker was using over 100% of my system resources consistently.
After looking up what md worker does, it appears to be a process spun off by spotlight to index your drive for quick searching. Even after a reboot, the fans would kick back in after a short idle time of maybe a minute. This process was seriously heating up the mac.