How can I convert my GarageBand tune to an mp3 without the static noise increasing?
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!