Almost all background noises on amplifiers with speakers are from bad cables or jacks..not being well plugged or the binds on a plug or a jack are loose and a wire is touching an amplified cable...so ..if it is a background ..check all inputs first ..cause the issue is on the amplifier not on the speakers...The cables that goes to speakers are not even screened cause there is no need..the issue have to be in the cable that gets from the source ..dvd or computer ..change that cable if you have a similar one ..and see the result ..or ..if it is a permanent cable ..check the plug ..and actually take the jack from teh source and see if the noise ceases ..if does you find the source..it is something wrong with the out plug of the computer (dvd ) or with the Jack ..or it is just a issue of not being well plugged in ..or has a damaged output in the computer and it needs to see the position .
Another possible source is a radio based device near by ..that interfeares with the amplifier...the way cell phones does ...so see what it may be this source .. ..but remember if it is a radio interference the noise will be slightly rhythmical ..from the waves of the radio signal....
By the way ..it may be also something related with the power source ..if you have something like a refrigerator plugged in the same line with this thing this may be the source ...or anything with a big electrical motor..like a blender or air conditioned unit ..