I was trained as an accident reconstructionist. With that training came a lot of understanding about tires and compounds. The rubber compound on a tire gets harder as it wears. Old tires don't have as much grip as new tires. That's why when you get brand new tires they sound extra quiet for the first few thousand miles. Now, as to why they are louder after rotation?, it could perhaps be two things. First, it could be that there is a big difference in weight from front to rear. You might try airing them down a bit. Second, the front end has adjustments that the rear doesn't have. (Toe and camber) which probably caused some cupping and wear. They'll probably get quieter as they wear back there.