I've been noticing a weird rubbing/grinding sound emanating from my drivetrain when I pedal. I just adjusted both derailleurs, with an experienced mechanic watching over me, so I don't think that the gearing is the issue. It seems to happen every half stroke, on both pedals, and is much louder when I'm putting lots of force on the pedals, such as uphill. I'm thinking that it might be a bottom bracket issue, maybe something with the bearings. Any advice or remedies? Any help is appreciated. Thanks!