Why does my staffie drawl when we are in the car?

i have a 5mth old staffie and sometimes not all the time when we travel in the car or van she drawls alot where its actually pouring out of her mouth she isnt ill with it and if we stop and take her out of the car it stops, the car isnt too hot and yes the windows are open so she isnt dehyrating....please help im a first time dog owner with 2 dogs cruz who is a staffie and tessa a4 mth old staffie cross who is terrified of the car bus van well anything with an engine,...