I bought a mountain bike new last fall, equipped with Shimano Deore front and rear derailleurs. The rear derailleur is now making a bit of noise during pedaling in some gears and responds slowly and a bit unpredictably when changing gears. I assume that it simply needs to be readjusted. This is the first bike that I've owned that wasn't a single speed - all previous bikes were for BMX racing and this is my first MTB. What typically needs to be done to a rear derailleur that's relatively new in order to get it back to the type of performance that it delivered when new? Thanks in advance.