How can I convert my GarageBand tune to an mp3 without the static noise increasing? After I export my GarageBand song and convert it to an mp3 and play it in iTunes, the background static noise is much louder than in Garageband. 

OS X El Captain 10.11.2 . Garageband 10.1.0

I want it to sound the same as it does in GarageBand. 

How can I convert tracks into an mp3 while reducing static noise in the file when played on iTunes?
I suspect this is not a file compression issue but rather an audio compression issue. By default, GarageBand has "Auto Normalize" turned on (Preferences > Advanced). This reduces the volume difference between the softest and loudest sounds in the mix on export, so that the playback of the finished audio is louder. The problem is, when you've got quiet background noise in your source files, normalizing will make it louder. Try turning off "Auto Normalize" and exporting again. The file format shouldn't matter.

Of course, the Jedi master-level fix for this problem would be to get a better recording (or better recording equipment) and to EQ/filter out the noise while mixing!