Trytone
With particularly strong bass notes (Cut Chemist-The Audience's Listening CD-track 2 & 3)at a robust (but not as loud as I might one day choose) volume.The problem only occurs with this type of bass note at this very healthy volume.
03-28-14: Mofimadness
I was a Magnepan dealer for many years. This is called "panel slap" and indeed does happen if the panel is overdriven or if the source material has really, really deep bass and played loudly. I've heard this happen numerous times.
It appears the solution to the problem is to be satisfied with a little lower volume, or replace the 86db Maggie with a speaker that has a higher sensitivity.