GSM buzz which only the person you are talking to can hear has been an issue with the GTA02. It has been discussed in several posts on the community mailing list. One early one is here . There is a fix for it, but it requires highly technical hardware skills. The big-C_rework_SOP_rc2.pdf, which describes the soldering required to fix the buzz, can be found here  as can a user's post of their result, from here .
Additionally Openmoko has support those offering a diy and encourage hardware volunteers. Fix it / buzz fix parties have been taking place.
There is still a problem though, as the wired mic causes buzz 
A bluetooth headset may be one solution.
- Hardware Issues - detailed discussion of problem and solutions.
- handheld-linux.com - Providers of the GSM buzz fix in the EU.
- [sdgsystems.com - Providers of the GSM buzz fix in the United States.
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