There are different opinions ranging from very loud to not worse then
my PC.
I've seen a report in this newsgroup from somebody who spend a weekend
to put rubber isolation inside the Blade to reduce the sound.

I've written a report about my Ultra 10 and how i made it silent (~18
db now - something you really can' t hear). Unforunately it's written
in german:
http://www.sonnenblen.de/index.php/topic,3551.0.html
I found the Ultra 10 to be much to noisy for a workstation. The Sun
Specs give allmost the same technical bell numbers. So i guess it will
be loud if you are sensitive to noise.

One more problem is that the Blade 1000/2000 do not contain normal
hardware parts, so it's almost impossible to create your own noise
reduction kit. And i don't know if there is one from Sun for the Blade
(i only heard about this option for Ultra 80/Ultra 60).