Thank you Alex….. FYI to forum members – Alex helped me today to find out why this occurred.
I sent him a copy of my show file and he noticed that the ST3 channel was turned up on mix 9/10 but not other mixes and this was the cause of the problem.
This did not occur to me because I had nothing connected to ST3. However, just having the volume up can cause the noise to occur whilst recording on older units. My unit is 2013 and the ST3 / noise issue has been a known problem and documented quite a few times here but I had assumed something had to be connected to ST3 – apparently not. Alex explained there is a modification available for this issue and I will contact my local distributor.
Excellent customer service!!