How can I fix the "pops, cracks and hissing" sounds from iPhone audio played on Ford Sync over USB? When I stream audio from my iPhone 4 into my Ford Sync audio system over bluetooth, the audio quality is excellent. However, if I plug in a USB cable to get access to the hands-free features, then the audio quality degrades almost immediately. Sound comes out with "hissing" sounds, pops and cracks. This is even more noticeable when listening to spoken word or podcasts. 

I have tried different USB cables and have verified that the same audio files do not play with the same defects thru the iPhone's speakers when using headphones. Also, the same files played over my Mac sound just fine. Any pointers would be appreciated. If you are experiencing this same problem with your Ford Sync system, please add a comment to know that at least I am not the only one experiencing this issue. Thanks.
I read a post about this before on the apple discussions board.  The upshot of it is there doesn't seem to be a permanent fix.  However, people have provided several workarounds of which some work for some people but none seem to work for everyone.  Here's a few things to try.  


Leave iPhone in Airplane mode while connected  
Turn off WiFi on the iPhone
Turn off 3G on the iPhone
Connect the cable while the device and stereo are off, then turn both on
Rebooting the iPhone  


I can't say from personal experience whether any of these actually work but anecdotal evidence from peoples experience in the thread linked to above would suggest they may.  I think it may be a hardware design issue that is probably due to interference from the USB cable when it's connected but many people on that thread say that it only started when they updated their iPhone 4 to iOS 4.1.  If you're on that version you could try updating to the latest version of the iOS software.  You could also wait until Apple release iOS 5 which is likely to be October 4th 2011 as they may have fixed it.