What was that?

Usually Leigh takes Mia to school in the morning, but today I took her. I was driving Leigh's car because that's where the car seat base is. I reversed out of our driveway, put the car in drive and started to move forward. As I did, I heard an eeeeeeeeeeeeeeerrrrrrrrrrrrrr. I stopped. The noise stopped. I drove forward. The noise started again. What followed was a series of starts and stops, while I tried to figure out where this noise was coming from.

The back seat.

Mia was growling every time the car moved forward. I thought it was some kind of engine trouble. Nope. Just a growling baby.