Bluetooth crackle when listening in car

It's possible, found this How to identify your SYNC version | SYNC | Official Ford Owner Site, but you may be wise to let your dealer do the update if any are required, it's not always straight forward, plus I would do the below first mainly because they are the easiest and involve less running around (and they are free, not sure if garage will charge for the privilege).

I would still try the "Forget and Re-pair" option first though, you could also Force Restart your iPhone (not sure if that will help in this case if Bluetooth is working fine for other connections, but still worth a try) If your iPhone, iPad, or iPod touch won't turn on or is frozen - Apple Support

Note Force Restart is not the same as just turning off/on. Follow the directions for "If your screen is black or frozen".

If non of the above improves the issue (and before you go to the dealer) you could hold tight and wait for iOS 12 which should be made available today, a complete overall of iOS could sort it all out.

Best of luck.

Do you experience the same problem when playing music on the iPhone with any other Bluetooth connected device, EarPods for example, also how does it sound over the iPhones speaker.

If all ok then go into the iPhone’s Bluetooth Settings and select “Forget The Device” (access via the “i”), remove iPhone from your cars system as well, then pair again and see how things are.

Try re-pairing as mentioned above, also have you tried connecting to another car system (a friends for example), if that works fine it might point the finger towards your cars audio as being somehow behind the problem.

You don't say what car or system you have, but if it's the type that gets updated via software, might one be outstanding for your model, usually it's done during the service but you could try contacting your dealer to see if any updates are outstanding.