So I already had one BDP-S5100 from Amazon, and bought another for the other room. The difference between them is STAGGERING. The first one I got a few months ago, and is nigh-on silent. The tray eject mechanism is quiet, playback noise is extremely low (and becomes "silent" the further you get into the movie, because of slower disc rotation).
The second BDP-S5100 I received just yesterday from Amazon is a whole other kettle of fish. The eject mechanism sounds very... cog-y. It's not smooth like the first one. You can tell it's running on cogs (which of course, it is). And then the noise it makes during playback is appalling. It does get quieter (though never actually quiet) as you skip through chapters of movies, but MAN is it loud.
So, for those who get a noisy BDP-S5100 (I know this is subjective, like... *sigh...* everything...), get another one. You might be surprised.