If you are hearing noticeable (i.e. about 5 feet away from the speaker) noise, the first step is to determine the source of the noise. The high-frequency noise and grounding flaws created by custom modifications or aging upstream components such as a CD player, D/A converter, or preamplifier may be audible as a loud hissing or buzzing noise. Typically humming noise is the result of grounding problem and hissing noise is caused by switching noise from external device (most likely from computer or its power supply). The first step is to start by disconnecting source devices one at a time until the noise disappears.Sometimes section of an old home might not have proper AC ground and the noise is coming from other household appliances. If you are unable to determine the source of the noise within the chain of audio components, and the noise can be heard by simply connecting the power amp and the speaker (without any upstream components), try other power outlets in the house and always use a quality power strip with noise filter.
Even if the source of the noise has been found, removing it is a challenge. Below are some common cases:
Computer switching noise passing through USB cable - the noise could be coming from the USB cable shield or ground pin. Try to use a powered USB hub and quality USB cable. There are high-end USB noise isolation devices but keep in mind that a cheaper solution might be to upgrade your computer.
Grounding conflict between components - this can be caused by poorly or incorrectly designed ground wiring that allowed noise from one component to affect another through the common ground in the system. Therefore a simple solution is to "cut off" or isolate the ground of the offending component from other parts of the system. This can be accomplished by using a "cheater plug" - an AC ground lifter or three-prong/two-prong adapter.
If the ground loop noise is caused by cable TV set-top-box, a ground loop isolator such as the Viewsonics VSIS-EU Cable TV Ground Loop Isolator (available on amazon.com)