Isn't is often pronounced [id-nt] /ˈɪd nt/ (Show IPA)
in the South Midland and Southern U.S., with [d] /d/
a stop consonant, substituting for [z] /z/
a sibilant. This substitution results from an assimilatory process in which the tip of the tongue, in anticipation of the articulatory position for the following [n] /n/
touches the upper alveolar ridge. This same process gives [wuhd-nt] /ˈwʌd nt/
for wasn't and [bid-nis] /ˈbɪd nɪs/
for business. These pronunciations are heard from speakers on all social levels.