I notice this in some people from Northern Illinois and Iowa and am wondering if this is a well documented phenomenon.
What most Americans would pronounce as “ar” is instead pronounced as something like “err” or “ærr”. This applies to words like are, car, card, hard, barn, yard, etc.
Edit: I do not mean to imply that this phenomenon is exclusive to Illinois and Iowa.
That’s probably a symptom of the Northern Cities Vowel Shift.
Wikipedia (see link above) says that in the Northern Cities Vowel Shift, /kɑr/ can shift to /kɐr/ or /kär/; /kɐr/ is halfway to being /kɜr/, as in cur. And since there isn’t any other r-influenced vowel around /kɐr/, one might easily hear it as cur. (Or maybe it goes farther than /kɐr/ in some regions.)
The Northern Cities Vowel Shift isn’t generally believed to merge car and cur, so probably either car isn’t shifted all the way to cur, or the vowel in cur itself moves to get out of its way. Or both.
When it’s not before a vowel, as in pot, /pɑt/, the vowel /ɑ/ goes to /a/ or /æ/, that is, pot is pronounced more like pat.