Well, after months, I have solved this problem! Thanks to this wonderful video of a tear down / put back together of a 306!
https://youtu.be/BjqPuwMb7nc(this video is made by a turtle owner. Is he on our forum?)
I realized that I was actually basically right- the white cover part of the filter was warped, so it wasn't sitting correctly and snapping into the impeller cover. After doing a complete tear down and putting it back together, I realized that all I need to do is pry up a little bit on the white cover each time I put the impeller cover back. This way, I can get it to snap into place.
Note that I'm talking about the problem of the impeller cap not staying on. The leak was fixed earlier by replacing the quick stop valve (after having also replaced the gasket). Any rattling I had was mainly due to overstuffing the filter baskets, I think.
Finally seems to not be doing anything annoying anymore