Bluetooth is simply a weak radio signal. Turn off your car's radio and any other audio devices. Like a previous poster suggested turn off your engine. Then try the bluetooth, has the crackling gone? If yes, turn on the engine, check crackling, keep on trying until you turn on something that interferes. If all else fails try it in a different car.
If you are still experiencing problems, try a different BT-enabled phone (different brand) with your SatNav to see if its that. In my experience, different brands of BT device work better together than others and it can just be 'luck' that your brand of mobile works better or worse with the SatNav than another.