Personally, I think it has something to do with the area of the brain affected by our Bipolar. To me it seems like an awakened sensitivity to the white noise of motors from electrical appliances. I notice the instrumental music when my car is idling at a red light. I have also noticed similar sounds when our old refrigerator was going out and it emitted a lot of extra motor noise. This is just my own guestimate - not anything scientific that I've read or heard.