Sorry for the delay in replying, today was the first opportunity I had to test it out.
I can be more specific now at least.
I unplugged my LAN cable so I am not connected, Wi-Fi is, of course, off.
I have changed nothing else (no buffer size increase etc) it is still set at 128 samples in the Fireface USB settings dialogue.
I am using the "WDM/KS" setting in the Driver Mode
I have a simple 16 bar loop running on one track with 8th notes using (what appears to be) one of the more simple presets in Rounds (Soft Criminal) There are no other tracks or effects running.
It is as simple as it gets!
Rounds was set at 88.2/96 kHz and when I change it to "Host/System (44100Hz)" it definitely gets better, but still does not go away completely.
Every time the Now cursor returns to the beginning of the loop I get an audible click/glitch. It also does it intermittently once or twice a cycle. It is only slight but if it is doing this with just one track I can't imagine how I am going to build a song!
Any ideas?
Thanks for your inputs.

Hello,
 
My first suggestion would be to see if you can use ASIO as the driver instead of WDM/KS. I would also take the NIC card one step further by disabling it in Control Panel ( even unplugged the driver is still active). You may be able to get rid of the glitch at the start by tweaking the Loop points slightly outward as long as it does not adversely affect the timing. For laughs modify the buffer to 512.  Let us know how it goes. Thanks.

Thanks for the quick response but it really shouldn't be necessary to "tweak the loop points", surely that is one of the basic functions of the program?
As for the ASIO driver, I keep changing back and forward between the 2 and am so confused as to which should be the best suited for the job in hand.
I did revert back to ASIO but it changed nothing.
I also put the loop on another track and solo'd it and it is still clicking on the loop return which would also eliminate Rounds as the cause.
I get an audible click every time I hit the space bar to stop/start the playback.
Thanks for the inputs.

Hello,
Let me clarify what I meant by tweaking. In working with some midi files, I have found that depending on where the note starts there are times when upon hitting transport I either get a glitch or click or the first not does not sound.  I addressed this by either moving the loop point to not be exactly on the start of the clip or by slightly lengthening the midi clip to I am assuming allow the first note to "hit". 
 
In regard to Rounds, did adjusting the buffer do anything?.. Disabling the nic card?

Update your audio interface drivers and firmware.
Update your NI software in their control center.
Use ASIO not WDM.
Try turning off 64 bit precision engine in Sonar.
Try turning off Intel Speedstep.

Thanks.

Try turning on the metronome during playback (in the options) and then muting it so you can't hear it. This is a workaround which stops the looping click in many cases. 
 
You should definitely be using the ASIO driver which came with your Babyface. RME drivers are superb and I doubt that it is causing the clicks. But there is a known issue with softsynths, looping and clicks. For some reason the metronome hack above seems to clear it up. If not, keep the metronome one and try a few buffer settings since that seems to have an effect as well - higher buffer settings may even make the problem worse (I think I recall someone else saying that in another thread). 

All drivers and firmware etc are already updated.
I'm not sure what you mean by "muting the metronome". I tried switching it on and off but nothing happened.
I'm using the ASIO drivers as you suggested but it is the same, maybe less "glitchy" but it's hard to tell.
Is this something I'm going to have to live with?