No, as stated above 3G is in a different frequency band and should not cause the GSM buzz buzz. I can tell when my phone drops out of a 3G zone but I get no noise when it is in one.
If the network you are on uses different frequency bands for GSM and 3G then the buzzing may be less noticeable, especially on the 1900 MHz bands, but if your 3G is on the 800 MHz bands it will still have buzzing. Most cities have 3G on 1900 MHz which is a very poor decision because its a much weaker frequency and you need more towers closer together to get the same coverage of half the towers on 800 MHz.
Data (3G) works best in the 800 MHz bands. When the 700mHz bands are used widely data speeds, coverage and in building signal strength will be MUCH better than the 800 MHz.
the iPhone 3g is capable of
UMTS/HSDPA (850, 1900, 2100 MHz) AT&T does not use 2100 MHz.
GSM/EDGE (850, 900, 1800, 1900 MHz)
your device can change frequencies while you are moving or standing still and you can step up or down from 2G to 3G and the buzzing can come and go with these changes. Most towers have dual antennas so they have 800 MHz bands and the 1900 as well.
