I was working on some music earlier and kept hearing a rumbling/rattling sound whenever the bass drum kicked, however I instinctively thought that it was related to the EQ or possibly to the drum sample itself. After several minutes of twiddling, I was able to isolate the noise to the left channel, and at that point I was pretty sure I was dealing with a damaged studio monitor. When I looked into the bass port, I noticed something odd, and upon further inspection, the culprit was found:
In case you can't make it out, it's one of those silica gel packets that are packed into boxes for controlling humidity. I've had these monitors for several months and never had an issue with the sound, so I'm assuming that the packet was lodged somewhere and it finally broke itself free.