My wife's 3GS had the buzzing noise during calls for ages and it got worse to the point could not really use the phone. It has been driving us nuts. Read loads on net and took it to Apple gurus who said £75 to change receiver but might not fix it and two other repair places that said economic right off so forget it.
Well I have now figured it out - there is a crack in he back rear housing case and I found that if I pushed on the crack I could get the buzzing to come and go. It must be the gold wires that people talk about making and losing contact with he frame.
At present a bit of of cello-tape over the crack has sorted the problem. If it comes back we may need to change the housing. But just to let others know for us it was these earthing wires, especially if you have cracked housing.
cjc