Case quieting.
Dynamat. Been there, done that. Good in car audio? yes. Computer noise? sorta.
Dynamat is not useful enough to completely cover the inside of your case with it. It *might* raise the temp of the inside by .5C, but thats not the issue. It WILL add 10-20 pounds to your case, and have little impact no noise.
Dynamat does GREAT work at absorbing vibration (its freaking asphalt!) But for normal noise deadening, it does nothing.
it can be used like this: Cut a small square out, about 20mmn larger than your fan, and stick it to the inside of your case. Cut out the hole in the middle, then use some grommets to aid in quieting the fan. Then rescrew the fan.
That's the most effective way to quiet your fans, aside from running them @ 7 volts.