What you experienced may be a baseline change when using noise cancellation. It happens with all noise cancelling headphones after long use (for me). I am NOT an authoritative source on it but what I heard was that the brain makes a baseline for the background / silence and when that noise is removed (active noise cancelled) then the brain recalibrates which may cause the same symptom as tinnitus as the brain tries to cancel it out but supposedly without the permanent damage. I consider it "possible" but I haven't seen hard science on it.
