How did the word "chunter" come about?

Solution 1:

It seems to come from Yorkshire dialect, with direct meaning of mutter.

Also, to me the connection with 'chant' looks interesting:

chant (v.)
late 14c., from O.Fr. chanter "to sing, celebrate" (12c.), from L. cantare, frequentative of canere "sing," from PIE base *kan- "to sing" (cf. Gk. eikanos "cock," O.E. hana "cock," both lit. "bird who sings for sunrise;" O.Ir. caniaid "sings," Welsh canu "sing"). The frequentative quality of the word was no longer felt in Latin, and by the time French emerged the word had entirely displaced canere. Related: Chanted; chanting. The noun is recorded from 1670s, from Fr., from L. cantus, from pp. stem of canere.

Solution 2:

I would hazard a guess at a modification of chant, which according to this link appeared in the 1600's from the french chanter: http://www.etymonline.com/index.php?term=chant