I have a 2012 klx250s and right after 2,000 km the sound of the engine changed. I came out of the mountains from a ride and it kind of backfired as the engine was decompressing on a downshift and it did a little backfire. Now the engine sounds deeper and a little rougher. Is this normal? I followed the advice of other riders and the dealer during the break in, but didn't follow the book. I have done two oil/oil filter changes. This is only my second season riding and first brand new bike so I think I'm a a little paranoid. Thanks for any feedback you can give.

 

Could be that your pipe connections have loosened up some. Check them, both at the head flange, and the joint between pipe and muffler.