1: Distance between the BW 900 headset and your mobile phone

Cracking static noise is significantly affected by the distance, and materials separating your BW 900 headset from your mobile phone.  To reduce this noise, place your mobile phone in the same side of the body as your Bluetooth headset. For example, if you wear the BW 900 headset in your right ear, make sure that your mobile phone or device is also to the right.

2: Surrounding objects emitting high radio frequencies

When using your Bluetooth headset stay away from object emitting high radio frequencies such as: Microwave ovens, Wifi links, wireless alarms, and other Bluetooth devices operating nearby.   

3. Mobil phones not supporting frequency hopping or eSCO

Older Mobil phones may not support frequency hopping nor eSCO; As a result, crackling noise will be more noticeable. To avoid this issue, make sure that your mobile phone supports frequency hopping and eSCO (all newer mobile phones usually does)