Which process is periodically writing to the disk?
You can get a better idea what's causing the activity with some of OS X's command-line tools:

sudo iotop -- display of what processes are doing the most I/O, updated every 5 secs.

sudo iosnoop -- shows I/O as it happens, including the process ID and process name doing the I/O, as well as data size, file path, etc

In both cases, you'll need to authenticate as an admin to run them, and use Control-C to exit.  They may not tell you precisely why the activity's happening, but they'll at least tell you what processes and files are involved, which usually narrows it down considerably.