Background noise

Hi,

I have created some pieces of music, however when I play them there is strange crackling noises in the background. This happens most when a brass instrument plays a particularly high or low note for a long period of time (like half and whole notes)

How can I remove this noise, and what causes it? I have tried compressing, leveling, and adjusting the bass and treble of the track on audacity, which has helped, however there is still some noise left.

Thanks in advance,

- MClasper