Sounds like a small adjustment needed. On the RD, there's a little black adjuster, right where the cable enters the derailleur. Unscrew it to tighten the cable, screw it in to loosen the cable. Try unscrewing it a quarter turn to see if you can get the chattering to go away and the shifting to improve. There's a whole set of steps to go through to set up a rear (or front) derailleur that I think any rider should able to go through, instead of hauling it down to the bike shop every time. Try the adjusters...if that doesn't work, go here and look on the right side for maintenance tips. Bookmark it...it's a great resource.