Axis Creaking - Finding a belt

My X-axis on the mill has started making creaking shuddering noises
when it starts and stops. Its not stick slip as it moves freely with
the hand wheel. When this shuddering noise occurs you don't feel it
through the table. The servo drives the axis screw with a cog belt
reduction and the encoder is coupled directly to the ball screw. The
belts are probably 10 years old and look pretty beat. Some of the
cogs are missing little chunks.

I have no idea what pitch they are as they came with the machine. The
manufacturer (Tree) wants about $50 a belt. They are about an inch
wide by about 18" and I don't see any markings. Can I take them to
Gates part counter and ask them for a replacement or am I going to
have to figure out what I've got and order it from somewhere? How do
you go about identifying a belt?

Roger

PS. Just ordered the Visual Mill/Alibre bundle. I chose Alibre
because it claims to be parametric. Hope they are more user frienly
than Mastercam and a lot of the others I've seen.