Found a solution!! I set out to find a solution today and to make a long story short, here's what I settled on: taping two small pieces of aluminum foil on the back of the iPhone in a specific location
(pic 1):
1) Cut out a 2" x 3" piece of aluminum foil.
2) Fold foil in half horizontally (foil is now 2" x 1.5").
3) Tape foil from the bottom right corner (on the back of the phone) up to the middle of the text "iPhone" (pic 2, pic 3).
4) No more buzz/static/popping sounds coming out of your speakers!
Another solution, to avoid using tape (although electrical tape leave no residue what-so-ever on the iPhone), is simply placing the iphone on a sheet of aluminum foil. This might be a better idea for people experiencing this problem in a static environment rather than a car (i.e. iPhone on office desk with nearby computer speakers). After discovering this solution, I also saw another potential solution online, using aluminum foil wrapped around the audio cable coming out of the phone. I haven't tested that method, but it looks much more obtrusive.
For me, other materials didn't work (at least not perfectly). I also tried various sizes of aluminum foil in various locations - you could probably get by with a smaller amount of foil, but this amount works 100% (so far at least).
I guess it should also be noted this solution could potentially work for other GSM phones with this issue (i.e. blackberries, sidekicks) - you'll just have to experiment with the size of the foil and its location.
Hope this helps!
Brian Cometa