|MadSci Network: Neuroscience|
What you are describing is the occlusion effect. Effectively sounds made in your head (chewing, breathing, talking, etc.) are normally transmitted through your skull and some actually exit your ear canal and are radiated into your surroundings. When you plug or cover your ears, some of these sounds are reflected back toward your ear drum. Low frequency sounds are especially prone to do this. You then hear these sounds louder than if your ears were uncovered.
You can find more information about this effect with regard to hearing aids (which often cause the same phenomenon) at http://www.hearingresearch.org/Dr.Ross/occlusion.htm
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