I had this issue when connecting my audio interface to an external power amp. I put a device, such as the  Behringer Micro HD400, in between my audio interface and the power amp and that solved the problem for me.

I had that problem and it took about 1 minute to fix.
I had hooked a second line of the tcElectronics firewire to the inbuilt amp & speakers in to one of the Sony LCD screens using your everyday RCA to RCA cable with appropriate adapters on each end. I pulled out the secondary out RCA leads and noise stopped.
So I went to my cable stash and got out 2 coax audio leads and used those leads to run the secondary to the LCD screen. Noise gone, the culprit was the RCA leads did not have shielding so picked up all RF from computer emission with the mouse being the major emitter.

It's Halloween isn't it?

+1 to VMW

Having good shielded cables will help. If you're using 1/4" to 1/8" adapters, you may want to replace that with a cable that has the correct plugs on it.

Well I made it to GC tonite and picked up some TRS-XLR cables but they have made zero difference. I did buy the $12 Hosa cables instead of teh $45! Mogamis. Not sure if that makes a difference or not. Craig Anderton suggested grounding the octa to the computer. I hadn't even noticed that there was a grounding screw on the back before. I just did that and, after trying a few different spots on the computer chassis, it definitely does make a significant improvement. I can still hear the noise but it is probably reduced by about 60-75%. Funny, I thought that when I finally hit upon the solution it would totally fix the problem. Either I need to find a better grounding site or look for other causes of interference.

Get an outlet tester. They cost about $5 at Home Depot. They look like a plug with 3 LEDs on it. It will tell you if your electrical outlet is wired correctly.

Good idea. I am wondering if my house ground isn't a bit sketchy!

Get an outlet tester. They cost about $5 at Home Depot. They look like a plug with 3 LEDs on it. It will tell you if your electrical outlet is wired correctly.

 
I had this proble where I used to live and it was a new house... every outlet and switch in the house had a loose ground wire. Make sure the outlet is NOT HOT, take the cover off and see if the ground wire is loose.(Sometimes a checker will test it OK even if the ground wire is loose... but it will still cause noise). If it is loose you may want to start checking all the outlets and switches because loose ground wires will cause electrical noise even if you aren't using them. Dimmer switches will make a lot of noise too. This all may be unrelated but I thought I'd post it. I hope you get it sorted out quick, I know this sort of thing can be really frustrating. Keep asking for help if you don't get it fixed... good people here!
 
Julien

I'm curious... do you have a power filtering power strip or UPS or are you plugged directly into the wall? If you are plugged directly into the wall I highly recommend at the very least getting a high quality surge protecting/power filtering power strip but if you have the dough a good uninterruptable power supply (with surge protection and filters) is even better. It may not solve all your problems and you should still get your house wiring checked (if things aren't grounded properly it's against code and a fire hazard as well as being potentially catastrophic for you gear) but they are worth it for the peace of mind alone. The good ones come with insurance for the equipment hooked up to it.

I picked up a nice Belkin power strip from Amazon for pretty cheap ($30 IIRC) which has $10,000 equipment insurance. Still gotta get a UPS but they are much pricier and weigh so much the shipping costs are prohibitive so until I can get my sorry arse to a proper computer store it ain't gonna happen.

Also I had a lot of noise problems until I picked up some balanced monitor cable but it seems you've already covered that. Cheers.

And people who do their own wiring without knowing what the heck they're doing really BEEP me off. People die from that kind of stuff.