Loud Music

This article doesn't link back to the original research, but the notion makes sense. Apparently there's a primitive part of the inner ear, the sacculus, that can respond to the rhythm of music louder than 90 decibels by triggering the release of endorphins. This would explain why volume can be such an intensifier of the effects of rhythm, but not so much the other elements of music. 

>>Neil Todd, an expert in the scientific study of music, explains that the sacculus seems to be part of a primitive hearing mechanism that has slowly been lost as humans have evolved. He said it has a connection to the part of the brain responsible for drives such as hunger, sex and hedonistic responses.<<