Dear Akry,
If you are sure that noise is from the interior, I hope this helps....It happened to my dad's pre-owned 1989 W126 300SE.Yours maybe the same case that the rattle noise was from the underneath of the front passenger seat....there was an electrical height adjustment that the nuts start to loose.That was funny that if you got a passenger seated or the seat was adjusted to the lowest position, that noise would be gone.You can get your mother-in-law to sit and see how it goes!!(be sure that don't let her talk too much!!) from DAVIS LO