This is more of a FYI because I had never heard of this problem before. Hopefully it will help someone out in the future. I have a 2008 wrx. I have always had a lot of sloppiness in my shifter since I bought the car (72k on it when bought) but a few months ago it started rattling as well. It seemed like it just happened out of nowhere one morning. Even a short shifter and polyurethane bushings didn't help, and the rattling was driving me nuts.
When I did my power steering pinion valve swap a couple weeks ago, I decided to look at the linkage because I couldn't find anything rattling from the shift assembly inside the car. I ended up finding the culprit. (See Attached Image)
The nut had loosened up and completely fallen off the bolt. The rattling was from the head of the bolt hitting the drive shaft. This bolt is also in a very tight space. It took a very carefully placed pair of vice grip pliers, a small box wrench, and a lot of patience to get the new lock nut on there and tightened down going about 1/10th of a turn at a time due to said tight spaces.
The lock nut size was M8 1.25 It took several different sizes to get the right one, because the bolt is impossible to remove and measure without removing the drive shaft and I couldn't find the specs for it anywhere. I had to just guess and see what we had at autozone, I got lucky and got the right one on the second try.
Now with this tightened back up, kartboy short shifter, and cobb bushings, the shifter is quiet, unwavering, and notchy, as it should be.