Is it just in lower gears? If it's your bigger cogs, then it might be that your b-screw needs adjustment to move it away from the cogs. If it's through the range, perhaps your RD tension could be tightened/loosened -- it's normal that you'll need a tune up in the first few hundred miles. Seems more likely to me that just a lube.

If you're into DIY, check out the RD "repair" section on parktool.com