My motor was making a strange noise, so I took the bottom of it off, and took the gear deepest in the box, and replaced it, but the gear I switched with was a little different. Does this change anything?
See this post: https://vexforum.com/t/differentiating-motors/43569/1
and follow the link to the vex robotics site.
Well if you took a gear out of the black side of the motor, then you will need to replace either the exact same gear or replace both with a different “set” of gears, see the link in @nenik 's post to learn about the “sets” of gears.