I'm a new owner of an X-400. It does indeed make a bit of noise. I have it outside of the case, for now, so I was able to listen to it closely with my ear next to it.
After an initial examination, I'd say there are at least two types of sounds from at least these two areas:
Area 1, near the power cord entry - the loudest part. Has a bit of a clicking/chirping sound (sounds kinda similar to the seek noise of a small HDD) and some buzzing.
Area 2, which is hard to pinpoint exactly, could be more in the middle - has a more constant very high pitched noise, similar to that of some TVs.
I couldn't hear a difference between CPU idle and load (LinX/Prime). But it's not a very power hungry CPU. (Might try with overclocking or gaming.)
Here's also a recording, made with a phone, so kinda bad quality with lots of additional noise (I raised the volume of the recording):http://www.mediafire.com/?cgy457dy1g6voo2
I moved the phone around the two areas. Around 8 and then around 18 seconds in, you might be able to hear the constant high pitch noise, which is more audible around area 2.
Bottom line: It's a bit of an annoyance, but it might be tolerable. I'll see how it performs once I put it inside the case (P183). I'm currently not using any power conditioner, but I'll definitely consider trying one.