Audacity files - background noise when converted to wav files

Hi,

I have used audacity to create some voice overs for storyline 2.  They sound fine in audacity, but when converted to wav file they have a faint buzzing in the background.

I have previously exported files using the same method and they were perfect.  I have played around with headphones but the same room, equipment etc were used.

Any suggestions?

Thanks, Shawna