Our brain has inbuilt noice cancellation Mechanism?

Most of us would've closed our ears with our fingers or hands at least once till date. And in doing so as a kid would have wondered on what the sound was that we could hear from the depth of our own ear! And as we grew some text book or general book that we read said that it was the sound of the blood traveling through the arteries/veins that made the sound. And yes if we were more enthusiastic, we would at times run hard to increase the heart beat and then check the sound from the ears growing louder when we closed ears after this exercise. And listen to the new sounds that got added such as the air passing through the nose to lungs and yes our heartbeat too was audible which was beating recklessly!

Now the best part is, Why don't we hear these sounds when our ears are open? A perfect noise cancellation mechanism was put into place in our audio processing unit in our brain when we were designed to walk the earth. So beautiful!

I know most of you know this. But jus thought to put it here as what is note worthy is that i never happen to notice this till date that this mechanism was actually there until i thought about a noise cancellation head phone. ;) Man, if god could foresee this detail a long back and had implemented a more complex audio processing unit for us! well, no need to mention BOSE would be out of business.... ;) Hehehehe...
Perfect example for "working out of the box".. the apple istyle... ;)