Deep - Dynamat doesn't absorb sound directly. What it does do is stops metal panels resonating. What I would do is put a big square in the middle of each panel of your case (on the inside obviously ). This will damp down any case resonance perfectly but you will still be able to hear the fans.
This is a response I got from someone whose done some work on noise reduction :
I've had tremendously good experience with carpets glued to the inside of
the case. Dynamat only helps to keep the vibration to a minimum. That's why
they are used on the under body of cars. They don't really absorb energy
from an accoustical wave. Foams are too soft. That makes carpets
perfect, it absorb almost the whole range of frequencies, which are hearable
for the human ear. The lower Hz can't be absorbed but the upper Hz, which
are created by fans, harddrives etc. are cut out by more than 50%.
Please look at my case to see an example for a com with carpets. If you're
willing to do some cutting and hot glueing (double sided tape also ok) with
carpets, e-mail me and I'll help you through the project.
[This message has been edited by deep (edited 04-05-2000).]