When I flush my toilet and the tank fills back up it makes a slight hissing sound. This stops when I tap the ball float. What is the problem? Is it a problem with the ball or is the water pressure to high or low? I am not sure..

Here are several ways to address this.
1- You can reduce the pressure at the angle stop **OR**
2- Shut the water off and remove the three screws on top of the ballcock, Pull the ball and stem assemble upand away, Now pry out the washer and reverse it. Seat it back in firmly.
Before you reassemble place a pan over the open ballcock and turn the water back on for a few seconds to flush out the supply, Reassemble and test. Good luck, Tom