I have a 2007 Mazda Axela, with the original factory stereo. It has an AUX cord fitted and if I plug this into my phone (or other device, eg other phones, raspberry pi) at the same time as charging through a USB/Cigarette lighter plug I get buzzing, crackling through the speakers. 

• Same problem with engine running or not. 
• Tried powering the USB 12v power adapter directly from the battery (eg using power lead running in the window) 
• Tried using a different USB 12v power adapter.
• Earthing the radio unit directly to the battery makes no difference (cable in the window again)
• Tried removing the band expander, no difference. 

It doesn't happen if I:

• Do not charge the phone or device. 
• Charge or power the device from a USB power brick (that itself is not charging from car). 
• Charge or power the device from the same USB 12v power adapter that causes issues, if I power that from a spare 12v battery disconnected from the car. 

I haven't yet tried running an inverter from the car 12v battery then using the inverter to power a wall phone charger to drop the 230vac down to 12vdc. 

Don't have an audio input device eg bluetooth to AUX that I can run directly off 12v. 

Any other suggestions? Is there another 12 > 5v USB adapter that would give cleaner power?