Hi there,
I have exactly the same problem. Before updating my iphone to 4.0.1, I had full bars at home, must of the time. After updating the phone, I haven't seen more than three bars.
I can hear that annoying buzzy noise at any call that exceeds 10 minutes (the noise is very similar to the noise that you can here when a cellphone is brought so close to a speaker). I believe this happens whenever my phone loses 3G and switches to edge. I am not sure if I had this problem before the updating the iphone, since I have got it just recently.
Anybody else with similar experience?