hissing in audio mix down -export

Can anybody help me? every time i export a song via audio mix down there is a hissing sound in the song that has been exported. There is no hiss or any interference on the actual track prior to export.

thanks for your help.

Its ok ive sussed it ! anyone else with this problem ensure that the sample rate is 44.1 and the bit depth 16 when exporting

I still have the problem. Any help please.