So my L200 is my first four-cylinder car and I've noticed with heavy throttle and passing 5k rpm it makes the entire cabin buzz and the engine noise seems to resonate throughout the car. I've heard that four-cylinder mills suffer secondary imbalance problems, so is that to blame?

 

Hard to tell without driving it. Maybe look up what year ecotec had balance shafts. I can say my 2.4L ION and Solstice are bloody smooth for 4 bangers. My youngest Daughter has a 2.2L 2007 ION and it seems fine to me.