Well, after much fiddling, I have come up with a solution of sorts. I got the Deltic working on the phone app, and reset CV8 to 8 as Hornby support advised, and on the HM7000 app the sounds all seemed to be working okay. Then I switched back to DCC (Select version 1.6) and immediately the din began all over again. Back to the drawing board. So I switched back to the app, went through all the sounds, and realised that the permanent din I was hearing on DCC was F14, flange squeal. So on the app I adjusted the volume of F14 to zero, switched back to DCC and low and behold, no din. Hopefully it has gone for good. I can happily live without a bit of flange squeal in my life. Still, it is a bit of a mystery why that sound should have been permanently On even when I tried to run on DCC without any sound. While in the app I also took the opportunity to turn down the volume of some of the other sounds (like F3, which is a very distorted horn sound).