The "nghiêm túc" you got it right but the "kẹt" is actually the sound of the door closingNice try though our language is difficult for other i know Melody
Oops haha thank you I wasn’t sure exactly what it was, so I looked through different translations. Thank you for the correction :)
http://www.mangago.zone/read-manga/my_one_and_only_cat/uu/iur_chapter-29/pg-12/
Kẹt = Rubbish, filthy (in other words he just feels trashy)
http://www.mangago.zone/read-manga/my_one_and_only_cat/uu/iur_chapter-29/pg-27/
Nghiêm túc = Serious