Have you checked if you have the same interpolation/resampling setting when you export as the one you work with?
To check the one you work with, go Options -> Audio Settings, and check the bottom right where it says "Resampling".
To check the one you export with, go to the export window, and check the Resampling there as well.
If they don't match, that could be the cause.
Also check and make sure you don't clip the output volume in the software (reset the output fader far up on the window, and adjust the masterfader in the mixer so it's no longer clipping, if it did).