I read through this thread before posting (it seems to be locked):
http://en.community.dell.com/owners-club/alienware/f/3746/p/19438386/20059912.aspx
I ordered an Alienware X51 (16G RAM with i7) recently with default video card. The unit has fan constantly running on 100% even on idle. There is no thermal controller to turn it down.
We really like the unit - it is very pretty - but the noise is unbearable. It is similar to having a vacuum cleaner (or an industrial server blade if you ever been to a server room) running on my desk...
I also checked online videos. Mine is louder than most of what people were posting. The difference is that it doesn't seem to be an issue with the fan, but either the motherboard or video card who is sending the fan signals to run (more likely the video card fan was the source of the sound). The sound "quality" of the fan seems normal, but extremely loud (not very high pitched).
Alienware autopsy passed for all components.
Unit was not overheating. In fact the wind coming out was rather cool.
I built a system last year with similar spec to the X51 I bought in a small mid-tower, it has pretty much no noise at all. (no fancy cooling systems, just the ones that came with the case) I understand that smaller units like this one have more powerful fans than a mid-tower, but it shouldn't be this loud. I can hardly hear my speakers with the machine is on.
After about 5 hours of Alienware experience (and headache due to loud noise), we decided to return it. From the research I did, we concluded that our unit is definitely defective (there are people with very quiet units of he same model). There are workarounds (such as manually setting fan speed with 3rd party software), but do not seem to worth the hassle.
I generally find DELL systems to be very reliable although they do occasionally fail on start (which is better than failing when warranty/return eligibility expire). This particular issue seem to be rather widespread though (posts surfacing in my search from 2010 to 2013). Does anyone know the actual frequency of people having this issue?
Other than this issue (which is a deal killer), the unit is very nice. The speed and size are both pretty good. The pricing is also fairly good (buying the components on regular market rice will cost about the same as buying the unit from dell). Hopefully this issue gets fixed.