Disable sound in Windows and see what happens.

(I am assuming your monitors/headphones are connected to you audio interface directly.)

The OS itself has nothing to do with the noise issue.
 
The problem is almost surely ground related.
A poor quality USB cable can induce noise (which sounds like "digital hash").
 
First, make sure your studio is powered via a single outlet.
Make sure to use balanced connections with decent quality balanced cables.
 
I'd start with nothing but the PC connected to your (audio) monitor setup.
Disconnect literally every component that's not absolutely necessary for the PC to run.
You need to establish a base-line "clean" signal... and build (add items one-at-a-time) from there.
 

Tremor
OK a quick update...

• The noise audio changes when using the PC sound card, but still there

I think everyone is picking up on the same thing here... and seeing your last bullet point, the below may "have to wait" but will throw it out for when the time comes.
 
If you go into "Change sound card settings" in Win7 with the audio interface online, are all "Realtek" devices disabled? You can right click that list to "Show Disabled Devices." This is definitely worth checking (but may already be done).
 
It seems you have already checked all hardware outside the box. Any other chords/cables to external hardware not taken from the loop yet (quick double-check to what Jim said above)?
 
Iobit's Advanced System Care (free version) is a nice utility to check numerous things on the machine. You can let that run itself, see what it finds, and go from there.
 
***Below is more of a "last resort," although is also good housekeeping when done properly*** 
Assuming this is potentially hardware internal to the box, it is driven by "something." In a situation that you are actively monitoring your machine, you can run Windows Task Manager and begin on the "Processes" tab and begin right-clicking processes and "End Process Tree" on things. Some are fairly self-descriptive, and you will eventually kill a main Win7 process and shut your system down if you exhaust this list (so do not do this while doing anything else). You may find a process using a piece of hardware that is unnecessary/conflicting on your machine. If you do happen to find something like this, the next stop is to disable it during boot in msconfig.exe (see below).
 
I have a truckload of programs/utilities disabled during boot in "msconfig.exe" (both the "Services" and "Startup" tabs), and when I discover more or these via the above this is where to disable them from auto-loading on boot. They can still be launched manually, but sadly... some which are manually loaded need to be disabled again this way, since they immediately add another registry entry for startup (Adobe being a prime offender of this).

Update..
 
Using only PC with its soundcard (saffire disconnected) and headphones - no noise ! Full volume on Youtube mp3 is silent apart from the track. That would seem to discount start-up items.
 
X3 doesn't see the sound card as an output device for some reason - not a listed device, so unable to output audio to it - strange.
 
next task is to remove the saffire completely from my rack system and connect it directly to the DAW with a short firewire cable and the same mains connection as the DAW.
 
Another slight problem has also arisen. When using the VSTs I occasionally get a short load burst loud of the VST I'm playing at the time.
 
My sanity is coming into question as well as ruining my music as I can't concentrate on anything other than the noise problems.
 
Aria did not supply a 5-pound computer hammer, but I'm getting close to needing one!!!!!

DSProductionz
I had the same issue for months.

This was a repetitive noise as described by the OP?  
 
For me it was random static.