Logic Pro 9 Disturbance in bounced audio in logic 9

there is a lot of disturbance in bounced file ( in mp3 128kbps) just like the disturbance from the mic ( a hissing sound).
When playing the track in the arrange window there is no disturbance. I am composing and playing the bounced file on the same headphones.
It is my first project on logic so I don't know much about the software.

Thanks in advance.:)