Chosen Solution
When cleaning the Wii sensor, I was putting it back together and found this piece left out. Don’t know where it goes but when I put the Wii back together again it makes a hissing sound, louder than usual. Thanks for any suggestions.
It is probably part of the mechanism for the casing in your third image
Disregard the red square as I grabbed this image from the ifixit wii teardown. The spring you speak of is in the red circle. This is 2 years too late, but hopefully it helps someone else who sees this.
