Thanks for reply, worth mentioning, but that isn't the issue. The ringing, buzzing is on the lower frequencies and is more of a chirping than a feedback. I have been on to Sonifex and am sending them so audio files of the same WAV file played on two different cards. It is rather baffling but my ear is drawn to it and wondered if anyone else had the same issue.
I mentioned in my earlier posts that I had failures of the Behringer FCA610 - I would avoid this product having had 4 failures and required them going back to Behringer. I can't fault Behringer's service in terms of warranty repair, however if you have mission critical and unmanned stations, avoid this product!