I had a similar problem a couple of years ago.  It turned out to be a defective audio interface that was allowing some of the output signal to bleed into the input signal, creating feedback.  I diagnosed the problem by temporarily selecting the onboard sound card as the output for the MAINS (eliminating any output going into the audio interface).  Maybe you could try the same thing to determine if your Q-C is defective.

Thanks for that Jim. I'm really beginning to think it is my Q-C.
 
I've poured over the Cakewalk article that Wizard links to and about feedback it says this:-
 

Note: This procedure does not eliminate feedback from your system, only the echo. If youexperience feedback, you have a feedback loop somewhere in your mixer setup.

 
So, no real help there then, and I think I've pretty much eliminated any Windows sound set-up issues.
 
I've also just tried a different, and shorter USB cable but it's not that.
 
I also noticed that, with the hi-z switch on, if I tap the Q-C you can hear that come through via the monitor. Weird.
 
And I've just added a Battery drum track to my latest test project. I used step sequencer to program a simple four to the floor beat, which I left playing. I then turned on Input Echo for the audio track and as I turned up the input gain on the Q-C you could hear the drum track coming through. Not as a clear drum track I hasten to add, but the meters "breathed," if you know what I mean?
 
I think I am going to have to make a trip to a Roland stockist and see if I can try out another QUAD-CAPTURE, or even something else for that matter.
 
Thanks again peeps. I think we've just about covered everything here for now, and until I can get my hands on a spare USB sound card I'm done with this...
 
As soon as I know anything more I will report back

An way you slice it, it seems to me there would have to be a hardware/driver problem with the Q-C, assuming it doesn't have a hardware switch or its own firmware/software mixer routing SONAR's output back to the input (a la Soundblaster's "Stereo Mix" monitoring mode).
 
Whatever driver mode you're currently using, you might try changing it as a troubleshooting step.

I'm looking at the QC and it doesn't seem to have any power supply options.
 
Bus powered? There you go... It's probably cross talk on the ground of the power supply.
 
Did you mention this already? Have you flicked the ground lift swtch?
 
How about the 48v phantom? Turn that off too... in case there's some issue in that step up and regulation circuit.
 
best regards,
mike

Try checking your settings in the windows mixer just to make sure nothing is awry with the selected recording device

Apart from that I'm outta ideas

Bibs

Good morning chaps
 
@  brundlefly, I've not tried a different driver mode yet. Might still do that (see below).
 
@ Mike, I've seen the ground lift and left it off because I didn't know what it was for (being a 53 yr old noob).
 
@ Wizard, I've looked, and looked again at the Windows mixer settings and as best as I can tell there is nothing awry there dood (see below).
 
Anyway, here's the latest:-
 
I thought I'd go into work a bit later this morning because I remembered late last night that I had one more option to try.
 
I hadn't tried the Q-C's 3-4 inputs which are the SPDIF connections on the back. I knew they were there, of course, but didn't see the point because I would not be using those for recording audio.
 
Then I thought, "you plonker Rodney!" I've still got my old Roland MMP-2 which is a two channel powered pre-amp and comes with a SPDIF output.
 
So, back in my test project I created a new track, set the input to "3-4 Left" on the Q-C, added GR5 with God's Love selected, plugged my SG into the MMP-2, turned on Input Echo and set the four-to-the-floor beat going on a loop.
 
No Feedback!
 
I would the input level up on the MMP-2 and I got loads of noise, but no feedback and no interference from the Battery drum track either.
 
Therefore, it must be something to do with the pre's circuit within the Q-C.
 
I need to try the ground lift switch next. That will be tonight because I need to get my ass off to work now.
 
I'll keep you posted as usual, and thanks again for all the replies, they have kept me sane LOL.