My bike only has 963 miles on it, and recently the drivetrain has been making a grinding sound in the lower gears of the cassette. It's not coming from the chain rubbing the front derailleur... no cross chaining here. I checked the chain for stretching around 800 miles and it didn't show much wear. All the cables are properly tensioned and I repacked the hubs 2 weeks ago. The grinding is the loudest under torque... like starting from a complete stop or going uphill. Any ideas?