Originally Posted by AnotherTennisProdigy
The sound comes from:
1. Loose grip and swing.
2. Clean hit at the sweet spot.
Flat serves and overheads are typically hit somewhat North of the sweet spot for maximum velocity. The local speed is significantly greater near the tip than at the sweet spot. The stringbed is still fairly responsive about midway between the sweet spot and the tip of the racket -- that is why flat serves/overheads are often hit there.