Just google "Mazda 6 speaker rattle" and you will see how widespread this issue is.

I am surprised that this has not been reported yet, but this issue affects ALL Mazda 6's from 2014 onwards. In my case with my specific 2016 Mazda 6, it was the passenger side door speaker. The problem is that there is a design defect in the door and speaker assembly, specifically the BOSE system which allows water to get into the speaker and RUST it until it breaks.

There has been no word on a recall for these speakers or a long term solution proposed. The solution given to me was simply to replace the speaker under warranty. This might solve the problem for a few years, but once the speaker gets wet and rusts again, it will fail. Some people have reported replacing this speaker 2-3 times in a few years. These are not cheap speakers either.

For a "Premium Audio System" this is a surprising design flaw. The problem is that the internals of the door are not weatherproof, meaning that all components present in the door can and will rust or break at some point in time. Hopefully something is done about this in the future.