Weird. I'm not super familiar with Mac OS but could it be your logs growing or something?
Discussion
Not sure. I've dug into many forums for a few days now.
I did find a folder called something like macmediaanaylitics and it was doing a folder "snapshot" at the size of ~70Mb, every 5-8 minutes..
Deleted that B and it did help a lot. But still growing so need to find more detailed, permanent solutions.