Unusual intermittent sounds??

Hi,
When playing back my score of about 40 bars, when the playback reaches bar 21 or 22, there can be heard what can only be likened to radio disturbance, it's as if I've too many parts playing at once, but when I just drop in at these bars mid-way through the score, it sounds fine. If I remove a part, one at a time, the sound interference also ends. Why does this happen, I'm using the iPad, and can it be stopped permanently?
Thanks a million
When playing back my score of about 40 bars, when the playback reaches bar 21 or 22, there can be heard what can only be likened to radio disturbance, it's as if I've too many parts playing at once, but when I just drop in at these bars mid-way through the score, it sounds fine. If I remove a part, one at a time, the sound interference also ends. Why does this happen, I'm using the iPad, and can it be stopped permanently?
Thanks a million