Hi Erik,
 
This is definitely a grounding problem. The obvious first step is to make sure everything in your recording chain is connected to the same circuit, or at least circuits that share an isolated ground. Check to make sure you don't have any noisy bits like neon beer signs plugged into that circuit while you're at it
 
My guess is the culprit is your Zed 16: Firewire interfaces introduce a separate path to ground that can produce the kind of digital hash you're hearing. The only solution in that case is, if you're currently plugging your guitar directly into the Zed 16, use a DI with its ground lifted instead. For microphone inputs, you may need to get some pin 1 lift adapters, but I rarely see this kind of problem with microphones - an unbalanced source like a guitar is much more prone to FireHash (trademark applied for  ).

If you are experiencing FireHash grounding problems, the noise is still there as long as the guitar is connected, even when you're not sitting in the offending positions - it's just quiet enough to not notice unless you really crank the system. It's being exaggerated when you position yourself appropriately in relation to the magnetic fields put out by your computer/monitor/console power supply.
 
There's also quite a bit of info about the Zed 16s having grounding issues with their internal firewire connection....seems like it was mostly earlier models, but Google "Zed 16 grounding problem"
 
So, happy hunting - ground problems can be a pain in the arse to track down if the quick and easy solutions don't work right off the bat.
 
<edited for clarity>

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Hi all,
Thanks for the replies.  Some more data:
The entire studio is on one circuit, I saw to that when putting it together.  It is also the only thing on its breaker.
There are no neon lights, florescents that could cause the noise.
My Zed is an early one and had the grounding problem initially.  However, I fixed it with the help of Mike G at Allen and Heath.  It has been silent and reliable since then.
 
My guitar is plugged into a Mesa Boogie Studio Pre.  The recording out is then going into the ZED via shielded cable.  
When the guitar volumn knob is turned all the way down (off) there is no noise.  Totally silent.  The noise is only there when the volume is up.  The noise is being picked up by the pickups.
 
Again, this noise was no present with my last computer, (or maybe it was, but just so faint that I did not notice it.)  Maybe the ZED's driver liked XP better than 8.1?  Could that possibly make a difference?
 
In my avatar image you can see the studio.  I typically sit right in front of the board when tracking.  the computer is at the bottom of the rack cabinet to the right of the right monitor.
 
When I hover the guitar over the ZED the "firehash" (I like the firehash moniker...)  does get a bit louder, but nothing like how loud is gets as I move the guitar towards the computer case.
 
Any more ideas.  I really appreciate everyone's input and time.
 
Thanks,
 
Erik
 
 
 

Try the same thing with a guitar loaded with EMG pickups.  If it still happens I'll be surprised. 

Yep, sounds like the guitar wiring plus lead is acting as a radio antenna, which picks up the radio frequency noise which then gets amplified along with the guitar signal. When you shut down a volume control you're basically grounding out the interference so it goes away.

As I said, my current PC has the same kind of issue. It's almost certainly nothing to do with ground loops (in my case the only connection between guitar and interface is an SM57 in front of the guitar speaker and everything is star earthed). I just get far enough away from the PC tower that the noise drops into the general noise floor.

EMGs might well solve the problem, but they also come with their own tone and a low impedance output which may or may not be a problem depending in what the guitar is plugged into. Some effects pedals (especially vintage style fuzzes) behave very differently when fed a low impedance signal.

Finding the right Pick ups is certainly a issue for those of us who sit that close to a computer and gear. I had to turn my back to the screen to play bass until I bought  Dimarzio's. All is super quiet now. I also swapped the stock wires to the output for a shielded cable.  
 
And that hi pitch screaming [or firehash,] is common and I was told it is the "pin 1 issue" 
It seems to be just certain computer set ups and the grounding of the pci cards or in most cases the USB ports. Bus powered USB audio interfaces are notorious. Sometimes special USB ( firewire?) cables are required to fix this. 

if there is a usb/firewire knob, try turning it to 6 o clock (half way)

If the interference noise changes as you move around, it is likely RF interference, not a grounding issue. Grounding issues tend to remain constant no matter where you move. Given that the noise reduces as you move away, the easiest solution is space. The more space between you and the source of the RF, the less noise. Second option is to put some shielding between you and your source. Even your body will work. Sometimes something as simply as putting your back to the computer can provide enough shielding.
 
Years ago CRTs would cause very similar problems. Same type of issue. The advent of flat screens helped out with this issue.
 
Good luck

Hi Erik,
 
I figured from your original post you had addressed electrical grounds...never hurts to mention it though. Without actually hearing the noise, I can't say for sure, but your description seems more like digital noise resulting from a grounding problem introduced by the firewire interface and an unbalanced input than an unshielded guitar pickup humming in the presence of an external magnetic field. It is certainly possible your guitar just isn't shielded very well. It's also possible that your new computer's power supply isn't as good as the one in your last machine, which could contribute to the problem if your guitar isn't fully shielded. Ultimately, you may not be able to do much beyond what people have suggested about just moving as far away as possible from the offending utensils.
 
Something to check out before ruling out ground issues altogether: I looked up the Mesa preamp, and it's not clear from their documentation whether its outputs are balanced or not. Try running the outputs into a DI with its ground switch lifted. Do NOT do what the Mesa manual says about AC lifting everything in your rack - that should only be done in cases of severe desperation, like a show starting in 5 minutes and a stage manager breathing down your neck
 
Another thing to check, if for no other reason than to reassure yourself that everything is good in your signal chain, is connect your guitar normally, but leave its volume down (obviously with the guitar placed somewhere away from where you experience the worst noise issues). Crank the input gain on the Zed ALL the way up, and record the noise. Look at the noise in an audio editor like Sound Forge or Audacity, and see if it's firehash or just good ol' white noise. Repeat the recording with your guitar volume up (muffle the strings with something), but don't play anything so you don't take your head off from the gain being cranked. Analyze it compared to the noise in the first step - that should tell you definitively whether it's a grounding problem or pickup shielding.
 
I'm pretty OCD about eliminating electrical noise in the systems I use and build, but I still get some buzz if I hold my basses directly over my console's power supply (a Yamaha CL5, so it's a fairly meaty external supply). My basses all have active Bartolinis, with good cavity shielding and good quality cabling throughout. Still, they're magnetic pickups, so..... At the end of the day, my solution is not to hover over the power supply while tracking - easy enough for me, but I have a lot more space than you do. You could always try optical pickups, too....
 
Hope this helps!

One other question while I'm at it, Erik: do you have the noise problem with microphone inputs, or just your guitar?