I just experienced this over the weekend on "my" father's day (Saturday) when the girls went last-minute shopping for the "actual" father's day. I was left alone (at last) for the first time since I got the LFM-1 and decided to put it through its paces.
I have two subs in my system, the LFM-1 and a Sunfire True Sub MK IV. I once had the Sunfire knock a sprinkler cover off the ceiling (before SMS-1), so I turned it down a bit this time.
I played Pink Floyd's Dark Side of the Moon SACD at -5db on the Outlaw 990 and thought I heard a faint rattle. My plasma monitor's attached speakers have been known to rattle, so I checked them and they were tight in place. I decided to really go for it and play the movie "Serenity" also at -5db. The beginning credits rumble is truly earth shaking and it did expose the LFM-1 as the source of the rattle.
I turned down the volume on the LFM-1 and kept replaying that scene until I had barely audible rattle (and before my ears started bleeding). Though I do not believe it is the plexiglass, since the rattle seemed to come from something "inside" the sub. Anyway, I rarely listen to anything at that level and other things start to rattle then anyway, so I can't blame the LFM-1 (too much). However, I am now even more impressed by the Sunfire True Sub.