They sound a little more nasal than the average American and they say words with ou a little differently than most Americans. Out and about. I don’t agree it is always aboot, many times it is a stress on the ou sound, an exaggeration of it. Wisconsin has some give aways like drinking from the bubbler (not all of WI uses bubbler) but that is not accent.
Other give away words regarding accent are creek and roof. Pronounced crick and ruff in some parts of the midwest.
Midwesterners also tend to not put the en suffix on words when they should (I can’t remember what part of speech that is?). For instance they will say, “I was bit by the mosquito.” Actually you were bitten.