Hi Greg, did you ever figure out what was causing the noise? Hopefully, you have by now.
Mine used to make the same noise when I would start it up cold (only in cold weather though) but would go away after the car warmed up. I dealt with it for 2 winters then did a bunch of research on the web and found this YouTube video:
The noise of that WRX sounded very much like the noise my Legacy was making, so I bought myself a timing belt kit from "theimportexperts" on e-bay. The kit included a new tensioner, idler gears, belt, etc. I swapped everything out in early July of 2012. That winter, the knocking noise was completely gone. So, it's possible that your tensioner is shot.
Give us an update as to whether you've fixed it or not.