Rhymes with Truck

Miles has always been a big fan of big trucks. One of his first "words" was "uh-guh" ("a truck") which he would say over & over when we saw a semi on the road. Eventually he called it a "guck" then a "big kuck." Now for some reason he calls it a "buck." I'm curious about all the incarnations of the same word, and at this point I'm just hoping that the next variation will continue to be G-rated....