Thanks for that, I did do a search for stuff like that and it didn't show up.
Looks like it is line noise as the problem went away completely when I reduced the input gain (no boost on either).
However it looks like the speech engine is randomly adjusting it back up to 91 (where it starts to have issues again). The speech engine seems to want the mic there for "green" levels in training (looking at your post here
https://voiceattack.com/SMF/index.php?topic=279.0) but this is clearly too high for VA as I then get the line artifact issues described in your post you linked. I'm going to have a few more goes at training the speech engine that it doesn't actually NEED to set the input levels that high!
Thanks again
Edit: Update, I've managed to "fool" the Windows voice recognition by putting boost up to max (+30 db) so it then auto adjusted to around the 70 mark on the main levels then closed out the trainer, removed the microphone boost and VA is now happy with 70-75 on the levels and speech engine is not auto adjusting levels in the background still.