There are a number of reasons that noise could occur and figure out the source is not always easy. Some of the most common sources are:
- groundloops which cause a hum
- Radio frequency (RF) signals and you can hear static, beeps and things and sometimes voices
- Thermal noise.
There is lots of information on groundloop troubleshooting I googled it and came up with this reference http://www.jensen-transformers.com/an/ts_guide.pdf
Electric guitars are good examples of audio devices that are sensitive to RF sources. The pickup and cable looks like an antennas. You change your position in the room and the sound changes, gets louder or quieter. It is better our worse at different times of day.
Thermal noise is a hiss and this usually results from either a defect componet (bad capacitor for example on a circuit board) or a poor gain structure due to a very weak signal source or the signal is turned down at some point and your than trying to amplify it later to get the amplitue back.
Practically there are some things you can do.
1st I'd switch the cables and inputs between the Rokits and the Logitechs. Does the noise follow the speaker. If the noise is in the logitechs after the switch than you can look at the cables and your sound card.
If the noise follows the Rokits I'd try pluging them into another signal source like a CD player. Use completely different cables. If the noise is still in the Rokits I'd take them to a dealer to be checked out (or have a friend come by who likes this sort of problem).
Cables are frequently the weak link so you could try replacing the cables.
Hope this helps.