Voiced "th" in "thank you"?
I have a friend, a native English speaker from Boston, MA, USA (I believe he is mostly Irish American), who is absolutely adamant that the first sound in "thank you" is voiced, rather than voiceless. Moreover, one of his college English professors (also a native English speaker, but not sure where from) agrees.
My friend acknowledges that all the dictionaries seem to say that the sound is voiceless; moreover, I was able to produce numerous examples from YouTube where a native speaker clearly uses the voiceless "th" (Matthew McConaughey, Jared Leto, Rebel Wilson, Victoria Beckham, Viola Davis, Dermot Morgan).
And still he---and his professor---insist that, in their daily lives, they have never heard the "th" in "thank you" pronounced as anything other than voiced.
My question is: is the voiced "th" in "thank you" something that is well-known and documented as a regional variant?
(I posted the same question at linguistics stackexchange, here. No actual answers yet, but from the comments it does seem to be the case that there is a subgroup of native speakers who do pronounce the "th" as voiced.)