I haven't had issue with this feature before, but I just updated to the latest version of Reaper and when I render to mp3 I'm getting audio artifacts.
With artifacts (mp3):
http://www.mediafire.com/listen/eu8k.../fly-drums.mp3
Without artifacts (ogg):
http://www.mediafire.com/listen/46ry.../fly-drums.ogg
The project file is super simple, it's just a wav drum track I exported from spark looped for a few seconds, no effects or anything that could explain this.
Edit: I went to LAME's page and downloaded the latest dll and now it seems to render properly
http://aegiscorp.free.fr/lame/