To silence my pc's I have done the following:

• installed one or more extra true low-noise fans,
• and added a miniature trim potentiometer of 470 ohm to each fan, so I can turn the RPMs down, and thus completely eliminate the fan noise.
• hung the hard drives in rubber suspension, so no vibrations reach the cabinet.
• added some padding foam to eliminate the whine from the hard drives.

My old HP C20 digital camera does not take close-up pictures very well, so for now I can not really show you any details, only something to give you a general idea of what has been done.

After replacing the fans, some vibration still came from the hard drives. So I attached some heavy wire strips from the top to the bottom of the case To support the HDs I cut out non-slide mat stripes, and made them wide enough to be rolled up, to ensure lasting strength. To ensure no static buildup, a ground wire was attached between one of the screw holes of each of the HDs and the chassis case.

As the new low-noise fans transport less air per minuet, and probably at a lower pressure, more holes were made, especially at the front, to let the cooler air into the case.
Using a larger drill and/or a Dremel bigger holes were also created in front of the fans.
By completely removing the metal in front of a fan, it seems like the air-flow is enhanced by 2-3 times.