Chain lube was what I was going to suggest too. I finally got around to switching my MTB to Chain-L; on my last ride it was so 'crunchy' I thought the drivetrain might have sand all in it. Quick clean with garden hose, wipe down with paper towel, apply Chain-L (and remove, clean, grease, and replace the RD pulleys), and my MTB is now as silent as a ninja.

So OP, I'm not quite sure from what you've told us, did we already eliminate the issue of chain lube? Because even a new bike, if you ride it a good lot for six weeks, could need re-lubing, if using mere mortal light-oil chain lubes.

You might also consider cleaning and greasing RD pulleys, but unless you rode through a lot of mud or something, I wouldn't think those would get fouled up in only six weeks. Other than that, I got nothin.