[niles]

They are normally spur gears. spur gears are only in contact one gear thooth at a time, then transmits the load over to the next come with an "impact"; this causes shock load to go thru the teeth, this is what causes the noise. Also since all the torque is supported by one tooth the stress is much higher; ie failure rates.
As opposed to a chain that has multiple teeth taking the same load. Also the slack in the chain flexs and dampens out some of the shock loads. This comes at the expense of less accurate timing and timing scatter.

If you want the best of all worlds there are hi-tech cog belts drives made by serveral folks(jesel, beesy bee?), that solves all the cons.
One tends to think of chains as crude; actyally they are extremely effecient, cheap and reliable.
gn