Sit in a silent place and then bring your hands close to your ears and cover your ears, you will start hearing some sound but if you remove your hands you don't hear it anymore. Can anyone explain physics of this?
The most likely explanation for the hissing noise you hear is ambient noise from around you , That is increased in amplitude according to the dimensions of the resonant cavity (your cupped hands ) so theoretically if you are in a real soundproof room this will not work , if you change the cavity size and distance of your cupped hands from your ears you can hear a slightly different pitched hiss . does that rings a bell ??
Try this take an empty preferably a thin walled metal mug (though a cold drink bottle will also work fine ) and some water , sit in a silent place keep the mug on a table now you must be able to hear the same hiss when you bring your ears close to the mug now pour in some water and try to hear again the pitch of the hiss(noise) must change for different levels of water in the mug ,The level/pitch of the sound will also vary depending on the angle and distance the cup is from your ear if it does it can prove that resonant cavity is the reason for this hiss sound.
Similar explanation goes for sea shells also seashells captures this ambient noise, which resonates inside the shell . The size and shape of the shell therefore has some effect on the sound you hear. Different shells sound different because different shells accentuate different frequencies.