Check the crank bolts. Retorqueing the crank bolts is probably the single most important step in a bike shop's standard 30-day new bike checkup. If the crank bolts weren't snugged up after the bike was assembled, the left crank in particular is likely to have worked its way loose on the bottom bracket spindle. (Most of the force you apply to the right crank goes into the chain; all of the force you apply to the left crank has to go through that left crank/spindle connection.) You can easily ruin a left crank (and possibly the spindle) by riding on it when the bolt's loose. It's a cheap fix now but expensive later.