Well, I already tried what you suggested since I've been messing with problems of this sort for ages. It is indeed a good way to determine the cause of the noise.
So far the most noise seems to come from my Mac G5. Mostly out of it's internal speaker (not an issue but I'm just letting you know), second there's quite a bit of noise coming out of it's headphone jack. THird the same kind of noise but quieter comes out of the main outs of my MOTU 828 firewire audio interface. There are several components to the noise. partially it sounds like weighted pink noise. There's also a slight beeping/ticking noise that really sounds like digital clock. This particular model comes with it's own optional digital I/O which cannot be turned off. I don't know what apple were thinking when they installed that monstrosity but they did and so there must be some dumb reason for it. Most pros who use Macs usually use high end PCI cards (Digidesign etc) or firewire interfaces. Who in the world would need an extra digital out that only seems to be causeing problems?
But whatever.
I tried using my G5 at a different location and virtually there was no noise. I even brought a few of my mixers to make sure it wasn't one of them (although I tried all four at my place. Same noise with all of them).
I suspect this odd problem has to do with weirdness caused by the optional digital I/O and it's digital clock. The rest of the problem has to do with some really sensitive circuitry that my G5 came with. Either Apple cheaped out and forgot to shield a few things properly or they didn't anticipate that it might be used in environments where the current isn't perfect.
Since I had no issues with this at another well powered location (a guy is using a separate power transformer and a few power conditioners connected to the outlets as well as has a well sheilded wiring) I've decided to pretty much do the same thing and install n independant power line running from it's own transformer. I will do it for my entire basement so this shit will never become an issue if I get any sensitive gear again.
On top of that I wil be taking my G5 to the shop to test it's grounding and everything inside. IF there's a way to kil that optional I/O I wil do that too.
Since you've asked my setup is pretty straight forward. Just for now. I wil connect more but here's what I've got now:
Motif ES8 (connected to the Mac via USB. No audio connections to the mixer yet)
Mac G5 (dual 2.7Ghz processor 1.25Ghz bus).
MOTU firewire interface (model 828)
Mackie 12/02
A pair of Tannoy system 600 self powered speakers.
Aside from those toys there's a modem connected to the Ethernet port of my G5. Disconnecting it didn't reduce the noise.
Okay. Thanks everyone. I shall now go rest. Will read ya tomorrow.
-ED-