Hi guys,
Finally solved this problem and it was all my fault. I open a new project and set up a series of aux inputs to use as bus channels. By default these channels are set as input 1&2. I pull up the faders to 0dB and of course I'm going to hear what's being inputted through these as well as the input channel itself. when I set the busses on the aux channels, hey presto, the extraneous sound disappears.
Thanks for your replies.