When playing music, it uses the A2DP Bluetooth profile, which uses a digital procedure that will correct for audio issues. It also has a better range and less issues with radio interference.
When using just standard audio for directions, the connection quality can cause this static and crackle. This is usually caused by some sort of interference, and can be an issue depending on where the car's Bluetooth receiver is located. You may try to relocate your phone to several locations to see if you can minimize the issue.