An annoying buzz which only the person you are talking to can hear has been an issue with the Neo FreeRunner. This defect is found in the versions A5 and A6 of the FreeRunner. The buzz will occur if these models are used with some radio cells which operate at a certain frequency. According to a German distributor it occurs for about 2% of all users. It has been discussed in several posts on the community mailing list. An early one is here: . There is a fix for it, but it requires highly technical hardware skills. The file 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, here .
Previously Openmoko had a program offering the big-C rework for owners of the FreeRunner in the United States and the EU. The Distributor Golden Delicious (see Distributors) will continue providing this service privately inside the EU. Furthermore some users are organizing Buzz Fix Parties where you can get your device fixed. The A7 model includes the Big-C rework by default.
Note that the "Big C" buzz fix only fixes the internal microphone; a wired headset mic will still cause buzz . A bluetooth headset may be one solution.