Depends on the pads.
When you replace brakes on a bike, it's the same as on a car - take the springs out and clean everything up before reassembly. Put a slight (SLIGHT!) film of brake grease on the backs of the pads, the contact points on the springs, and on the contact points on the retainer pins so everything slides easy. This will also help mute any vibration/noise.
Beyond that, if you replaced the standard pads with sintered pads, they have been known to buzz and squeak a bit. But they seem to stop better and make less dust, so a little noise is a fair tradeoff. IMHO, anyway.
