If you do not already know how, here is a good how to for getting the trim panels off:
2012 Focus SE Hatchback Sport - Changing stock speakers
Be careful not to put putty where it could fall to the inside of the door shell. It would be a mess trying to clean that stuff out of a window regulator.
Good point about the reflectors I remember how easily they slid out when I took off my trim panels.
Good point about banging (beat lightly) to simulate vibrations.
What I suggested would probably only work to stop the trim panels from vibration caused by the sound system. It would be a bummer to pull the trim panels and not take that opportunity to fix and other rattles that may exist.
And yes I am sure the dealer probably would fix that - or try anyway. I really do not like having to go to the dealer, it is a hassle (take off work to sit around and wait) and so many times you get the NTF remark. I think if someone posts an inquiry here they are looking for a way to do it themselves. I give them credit for knowing in advance they can take it to the dealer.
Plus I like to do that sort of a thing as a hobby, you know putter around in the garage. I would rather make repairs I can do my self rather than let someone else work on my vehicle. I realize I am taking the risk of messing something up. Wouldn’t be the first time, won’t be the last.