Hi all...
I posted this problem a couple of months back but still looking for a solution.
There is background bursts of low level noise all the time coming from through my A/D Firewire interface.
I thought it might be the Firewire 410, so changed it out for my Behringer - no different. I now have a Focusrite Saphire pro 40 in place, but again no different, so reckon it's not the A/D.
Since the DAW PC onboard Firewire chipset is not a TI one, I've disabled it and installed a Startech 4 port Firewire card which does use the TI chipset instead.
Again, no difference whatsoever. 
Really struggling to know what to do or try next. It might be my imagination, but the problem seems to have got worse since I upgraded the DAW from 32 to 64 bit Windows 7.
If it's coming from within the PC operation itself, how on earth do I stop it or at least cut it down?
Any and all ideas welcome!!

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

This could be so many things, so isolating each item in the chain is about the only way to troubleshoot. Do you get it with no inputs connected and the transport idle? You seem to have focused on inputs only, but have you checked connections/possible grounds in outputs? Direct monitoring with headphones versus monitors, etc.?

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.

Background means that it occurs when you are not doing anything at all with the computer when it  happens, and as far as you can tell there are no active computational activities going on 
Bursts means that the noise happens suddenly between intervals of silence followed shortly by intervals of silence
Noise unfortunately does not mean much at all other than that it is an irregular waveform that is apparently within the audible range. An oscilloscope might clarify what the predominant frequency of the bursts is. If it is a simple multiple of your line current frequency, then you would look for a source of "hum." If you have recorded the noise, posting a link to it here might give the golden ears (not including myself) who frequent this forum a clue.
 
Most often this kind of symptom is due to electrical problems in the analog realm, due to the wiring of the peripherals picking it up. It is possible that this electrical field is being generated inside the computer case, but it then has to be transferred to the analog realm and picked up by a transducer (microphone, A/D etc.) in order to become digitized and show up on your digital recording. If it does not show up in your digital recording, then you can just ignore it. 
 
Since replacing your computer power supply motherboard etc. one at a time can get expensive, and since the problem is more often with external wiring that would be the place to start. If it is audible while monitoring but not present in the recording, then look in the wiring to the speakers or headphones. Some components are notorious for broadcasting electromagnetic interference, like light dimmers, flourescents, motors, switches and wall wort transformers. 
 
http://ethanwiner.com/dimmers.html

http://www.soundonsound.com/sos/jul05/articles/qa0705_1.htm
 

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

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.
 

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? 

Any chance there's demo software, or software that hasn't been properly authorized yet, that emits bursts of white noise on occasion? That's a popular technique to prevent people from using demos for the real thing...

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.