Whispers, taps and tingles — what is ASMR?

Have you heard of ASMR? Whispery, clicky, crinkly videos are massive on YouTube - racking up millions of views. The idea is that these sounds elicit a certain tingly, calming sensation in some people. So what is ASMR and what does the science tell us about it? Is it real … or pseudoscience? And why do some people get the opposite reaction – irritation rather than these pleasant tingles?