Why do we hear a roaring or humming sound when an empty bottle is held close to our ear ?
When I hold a flask close to my ears, I hear like a sound which coming from its interior.
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The usual way that this phenomenon is described is
when you hold a sea shell up to your ear, you can hear the sea
What actually happens, according to this link, is that the vessel acts as a resonator / reflector for ambient noise - sounds that already present in the environment are amplified and stand out more.
The "you hear your blood" myth is just that - a myth. Although (healthy) human ears are exquisitely sensitive... see my answer to a recent question on the subject of just how little air movement can be perceived as noise.
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$\begingroup$ Testing with bottles of different wall thickness confirmed: I don't know what OP is talking about $\endgroup$– npstCommented Nov 22, 2014 at 3:46
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$\begingroup$ @npst - did you try just cupping your hands and holding them over your ears (without closing off the space completely)? Do that in a slightly noisy environment (some background fans, etc) and you'll hear a rushing sound, most likely. $\endgroup$– FlorisCommented Nov 22, 2014 at 4:01
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$\begingroup$ Yes, I meant I can't find any difference between different bottles, filled or not when the ear is closed off. $\endgroup$– npstCommented Nov 22, 2014 at 4:09