In the past, I’ve tried both aluminum foil and cigarette foil paper to block the iPhone’s GSM/Data interference from entering my car speakers (also notorious with computer speakers). Both worked okay, blocking most, but not all, of the speaker noise (I’d say about 80% was blocked).
Recently, I found a solution that blocks 99% of GSM noise, Static Free Bags (like the kind a new hard drive or RAM is packaged in):
If you’re using your iPhone at home and experience the interference sound, simply resting your iPhone on one of these bag will quite all noise.
Since my problem is in the car, I cut out an iPhone sized amount of Static Free material from an old hard drive bag and put in between my iPhone and Otterbox:
One layer is enough to stop ALL interference sounds from entering my car audio system. This solution works better than aluminum foil and cigarette paper in terms of blocking GSM, but just like before, it also slightly diminishes bluetooth reception (more static) and probably some cell signal too.
The bluetooth is only a problem when I’m in my home — where I have lots of wireless signals (telephone, bluetooth keyboard/mouse, wifi, etc) in a small space — not in my car, where I need it most. I only assume cell reception is hindered because bluetooth is, but it certainly hasn’t been noticeable.
For me, it’s worth the slightly diminished bluetooth quality for uninterrupted music listening enjoyment!