Hi, After visiting Death Valley (where it is extremely quiet) and after trying earplugs... I have noticed that there is a humming sound in my head when outside noise is blocked. So this got me thinking about helicopters and how they use a cancelling soundwave to block out the noise generated by the chopper. if you take 2 opposite sound waves and you combine them together... giving you destructive interference... then you come out with no noise (in theory...right?) so, what I was thinking... was that because I live in the city...and there is always a noise murmor in the city or busy metropolitan areas... our brain has created an opposite wave to the murmor which is meant to cancel the noise out. so, when we are in a quiet area... there is no wave to cancel out what our brain is generating (there is no city noise to destructively interfere with what our brain generated)... so therefore we can hear the wave that our brain generated. this is just a HS physics student's crazy theory... any input is very very welcome! thank you!