Low level noise is driving me mad ...what the hell is causing it?

Are you sure it's not a vintage VST that's introducing the noise like the tape emulator?

Is it getting printed to your tracks?
 
Anyway one possible and important thing to check is make sure you do not have audio cables anywhere near power cables or anything devices with power running through them (especially you monitor screen or your reference monitors/spekers/poweramps/etc). I recently created an elaborate system for running patch/mic cables coming from my guitar amps, mixing console, etc up and across my walls so they are as far away from anything with significant juice running through them (like my flatscreen monitor and the top of my guitar amps). It helped get rid of a lot of hum.
 
The other thing is to make sure you are using quality cables and that whenever possible you use "balanced" and/or "shielded" cables which reduce noise interference. Also I had a problem with some of my "brand new" patch cables where the darned things were scratchy and causing hums/cutouts. I unscrewed the connection sleeves and the way they had been wired/soldered was a total mess. I could have resoldered them but I managed to just slip little pieces of electrical tape to keep the frayed copper strands from touching crap they shouldn't have been and voila! Nice clean patch cables.
 
And as was mentioned above if you have a bunch of effects, particularly the new Prochannel Tape Sim, disable them one by one to see if they are causing noise. That noise will NOT be printed on to your tracks until you bounce/mixdown. 
 
Other than that make sure you are using good powerbars for all your equipment and that the outlets in your studio are properly grounded. If you are using electric instruments such as a guitar try moving around your room a bit with your headphones on monitoring the signal to see if you can find a "quiet" spot and then record playing in that position. That will also give you some clues as to what equipment in your studio may be causing hum through your pickups and whatnot.
 
As far as I know I don't think digital cabling such as USB or Firewire can cause audible hum because it is raw data (not audio) being transmitted but honestly I do not know. I personally would look in the analog part of the chain before blaming the digital devices and connections BUT it doesn't hurt to have quality cables to hook your interface up to your computer.
 
Cheers.

Cellphone near your workstation? Those things put out a lot of RF noise. Definitely make sure it's not just your monitoring system.

Bristol_Jonesey
Good post slaratabartfast.
 
I'd start by disconnecting EVERYTHING and reconnect your system slowly, one piece at a time and listen critically after each connection. Monitor LOUD.
Connect all mains apparatus before looking at Audio/Midi/Digital.
Power up your Monitors first, then computer, then console/desk if you're using one.
 

This...
Do you have balanced and properly shielded cables for your monitors? What happens when you plug in headphones? 

I had an issue once of this nature. It nagged at me and made me mad.

Somewhere along the experimentation route to quell it I put a $0.50 three to two prong grounding adapter on one of my power lines. It cleared immediately. Years later that little charmer still works.

Problem solved in my case for 50 cents.

Wow! First off, many thanks to all who have contributed, really appreciated.
 
To clear up a few suggestions. It's not residual demo white noise, it's too low level for that although the noise does have a "white noise" sound to it.
The noise bursts are "regular" i.e.about 2 seconds apart and each lasts for about 500mS which would seem to indicate that they are being "clocked" somehow - the PC perhaps?
The noise is recorded if I open a clean track with no other audio - the noise is too low to be heard on a track with audio on it unless there is a silent period in the track.
I can also hear noise while moving the mouse - only while it is moving.
To try and minimise potential external sources, only the A/D is connected to the DAW and I am monitoring the noise directly on the headphone output of the Sapphire.
 
All the points about earth/ground  loops are noted and I have a made up a socket box with a switch to remove the power earth connections individually. All my outboard expanders are powered on of course so maybe I need to remove the power from them all.
 
Another point I had overlooked is that the DAW and monitor run from a UPS, and obviously I need to remove that from the equation as well.
 
I do have a scope to look at the noise waveform. I can also record a few seconds of the noise, but not quite sure how to put that into the forum???
 
Again many many thanks to you all for your suggestions. Over the holiday period I will have a bit more "exclusive" time to investigate and report back.
Happy new year to you all too!!
 
 
 
 

You are connected to the UPS via USB for clean shutdown when the battery runs out..?

Hmmm

If you take the computer out of the equation, do you still hear the noise?

Use DPCLatency checker and Latmon Latency monitor.
If there is a process that is causing it, you should be able to get an idea via those programs, it may well show up as spikes in the DPC prog.

Other than that there could be a dodgy component on your motherboard or in the psu.

FWIW, At one point it was useful for me to create a new user profile. My old user profile ended up slightly corrupted with the DRM stuff. IDK what did it, by the guy from Syntorial.com suggested it as a fix and it did the trick for me. 
 
I'm meaning at the windows level, I created a new login account.

Long shot but if I have an input channel unmuted which has phantom power going through it, I get pretty weird noises that end up on my recording. Might be worth checking all your unused inputs are muted?

I just wanted to add encouragement or a sanity check to this discussion.

I've dealt with noise from various contributing sources over the years. There were times that I thought I couldn't lick a particular issue and settled, accepting that as the fate of my setup. But through persistence I did eliminate them all.

I think it's helpful to know that "normal" for a recording setup can and should mean no noise at all. No hum. Just no sound unless you touch an instrument or play back something.

It can seem acceptable to live with a hum or other noise, but it's a helpful sanity/reality check to hear from others that zero noise is possible and should be normal if you can get there.

I've heard this with a wireless mouse (as mentioned above).  I only use a corded mouse now.

 
I wouldn't go disconnecting ground wires on electrical equipment, ie the third prong of the mains AC supply, especially in the UK with your 240VAC lines.  The ground is there for a reason and disabling it puts your safety and the safety of others at risk.
 
I agree with what Bristol Jonesey suggests.  Does you computer have onboard sound ?  If so, I'd start with just Windows sounds, onboard sound and your monitors.  If your monitors are powered, try them on their own completely disconnected from everything else.
 
 

As described this is repetitive and consistent.
Has to be a timed source.
Some control signal, ping, testing for a condition.
(Capacitor charging/discharging?)
Network adapter checking for connection
software updates,....
 
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Reference: my powered speakers do NOT have a third rail on the plug.
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Had an issue a couple of years ago with a static sorta noise. Did all the stuff I could to isolate. Finally called the power company. First thing the dude did was pull out his very big screw driver, open the mains box and tighten the biggest screws, and walla.