Most likely the BB is moving ever so slightly within the frame/shell. We see this very frequently and more often with AL framed bikes. The suggested fix is straightforward. Remove the crank and then the BB from the bike. Look at all surfaces and the spin condition of the BB. Clean all surfaces. Only replace the BB if it spins roughly, notchy or has bearing slop. On reassembly coat the threaded portions of the shell and the outside of the BB (including the LH "cup") with a HD grease. Tighten to spec. Clean off the BB spindle tapers of any lube and reattach the arms. Lube the retaining bolts and torque to spec. Ride the bike and re torque the retaining bolts.

Do this once a year for average use. And I suggest that you take a look at the wheel bearings for their smooth spin. It's common for shops to not preform a hub bearing adjustment on the bike's initial assembly and the factory adjustments are frequently tighter then best. Andy.