Is it always the same section of the same songs, or is it random? Radio/CD/AUX?
Sounds like a loose connection. My guess would be at the crossover. If it's while using the aux input, a loose connection there could also cause that sound.
MODS: PW TMIC, Cobb catted DP, HKS cat-back, AVO filter, Bren e-tune; Konis/Epics, Advan RCII