There's a very recent paper on PeerJ (hooray, open access), perhaps not what one would call "scientific backing" in the strongest sense, but more a study aiming to establish the scope of the phenomenon, and relate it to other aspects of perceptual experience - through survey of self-reported ASMR experiencers: Barratt & Davis, 2015

While ASMR appears to be a genuine, relatively prevalent perceptual experience, the exact nature of the phenomenon is still unknown....

Full survey data is also provided as supplemental information (see the link above) in case anyone wants to do some deeper digging.

Could be related to skin orgasms. Some people have brought it in relation with foreplay, cuddling and delousing. Perhaps the closeness of the sounds evoke strong associations with being actually touched resulting in something like a laughter response to being tickled (which could be thought of as a signaling that there is no danger from a spider crawling down your neck). Perhaps the high spectra in whispering make it even related to chills we receive from high-pitched noises, which is possibly related to teeth maintenance. I'm not aware of any research.