The number of links in a chain has no bearing on noise.
If it clicks in all gears, it could be a stiff link, but if REI looked at the bike, a stiff link would normally show up in the repair stand, and it would click even without pressure.
If it is noisy only in one gear, and it is the gear you most frequently ride in, it is likely the cog of the cassette is worn to match the old chain. It is not enough wear to cause skipping, but enough to be noisier under pressure. Depending on the position, the cog may or may not be able to be replaced individually.
On the other hand, if the noise is coming from the front ("The sound is mostly coming from the motor cog area."), perhaps you are describing a rubbing front derailleur? In the repair stand it won't make noise because without pedal pressure, the frame does not flex. Under load the frame and crankset can move laterally, causing the chain to rub against the outside derailleur plate. But this would have happened with the old chain as well.
Another thing to check...especially if you broke a chain...is the rear derailleur hanger alignment. A misaligned derailleur can cause noise.