The noise of your computer is caused by two fans. The feed fan(with the blow off opening at the back) and the processor fan in the housing).

In order to reduce this noise you can buy special silent fans in combination with placing Vibraflex on the inner side of the computer. When this is not an option, then you can do the following:

The PSU fan is the most difficult to damp because it blows out of opening at the back. Noise is then developed outside the housing. This noise also has to be damped outside the housing.

When your computer is placed in a kind of housing, (for example a separate pocket of your bureau) then you can cover this with Acusticab. This will absorb the noise to a large degree. When the computer is placed on an open space you can try to build a labyrinth with Acusticab.

In combination with the Acusticab you must anti-drum your computer on the inner side with Vibraflex Normal; this prevents vibrations (which the fan generates) being transferred to the surrounding (transformed into noise)