Have just discovered something – when I ‘preview media source channel’ the first two channels have the interference, then channel 3 is absolutely fine, the other channels (up to 8) are silent.
When I uncheck the first 2 channels and keep channel 3 selected, the clip plays silently in the timeline.
If I select ‘3’ in the ‘active channels per clip box, only the first two appear in the timeline track.
Not sure if that’s helpful, but at least I know the audio is there. Just need to find a way to select it.