Strange noise problem with music layer after replacing the music file using script in CS6

Hey guys,

I got a strange problem. Sometime when I import a music file, or replace a music footage with another music file using the "replace()" or "importFile()" method, the music layer added one second noise at the beginning. like this,

Does anyone know what's going on here?

I'm using After Effects CS6 windows Server 2008 64bit.

Thanks in advance!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!