These are fascinating observations. I hadn’t heard of the Wim Hoff method but since reading your report I have been learning about it. I may try to include that in my meditations. I have been using my QO for about three months and I’ve noticed a similar sound when lying down. Exactly like a low pass filter. I’ve also noticed the crackling noises, but only after I have been listening deeply for ten minutes. Perhaps I’ll hear more when I try the breathing exercises. Thank you for sharing your experiences!