Sporkupine

So I'm pretty technical with computers, but find myself shockingly ignorant when it comes to troubleshooting audio. I have a set of Promedia Ultra 5.1s, which are about five years old. Within the last week or so, I've noticed they've started hissing pretty badly... sometimes. Last Saturday it was hissing badly, I started looking around online for tips, tried a few things with windows settings, line-in, etc... and nothing worked. But then, out of nowhere, it stopped on its own. So today the speakers are running fine, no hissing when I get home, or while I'm watching a TV show on my computer. But then shortly after the show finishes, the hissing begins anew. I started looking for solutions again... I won't bore you with the software settings I tried, because I eventually found out that even with no signal coming from the computer, the hissing continues. Physically disconnecting the comptuer-speaker audio cables does not have an effect. The volume control does not affect the noise level of the hissing (I can turn up the song loud enough to drown out the hiss, but it must be pretty loud to do that). Someone suggested it could be some interference from my cell phone. Even though I have been using this office configuration for 2 years now, I turned off my cell phone with no luck. I turned off my monitors with no change. I've physcally disconnected the power from the speaker a few times to see if that could help, with no change once it's plugged back in. I checked the audio wire connections on both ends, they all look good (I'm using a thick gauge copper wire encased in a clear plastic). I currently only have the front speakers connected (R, L, C) and the hissing is coming from each one. I'm using 2.1 audio settings and have been without incident for 2 years. The noise is a static white-noise, and increases in volume and frequency when I power them on, hitting "full volume" within a few seconds. Occassionally, without reason, the noise will alter slightly... but just to a different sounding white-noise. As I was writing this the volume started decreasing, almost started to sputter out... but to my disappointment, started back up again with just a different frequency and slightly lower volume. The hissing is pretty much driving me nuts, so any suggestions, tips, or advice you can give me, would be really appreciated. TLDR: loud hissing even with no signal coming from my comptuer. Tried turning off cell phone, monitor... still hissed. Hissing volume independent of control volume. Hissing has gone away randomly and returned equally as randomly.