Delays can be noisy too.  Very noisy!  I always export to WAV and if I want to make it an MP3 I'll use Audacity later but certain types of delays or flangers create  a "breathing" sound even in the point between any silence you have at the beginning of the project until the music starts to play.  As Bitflipper says, it's best to do an export from a fresh and non played launch.  I also find it a good idea to keep at least three measures empty before starting the music.  This gives the reverb/delay tail time to dissipate before the music starts.
 
John B.