crc4 appears out of no where in system.conf

Hello folks!

We have FreePBX 2.8.0.4 and asterisk 1.6.2.10
We have two E1 cards and the we are having a rather strange problem, every once in a while the agents hear noise in the line.
When we check the configuration in system.conf we see instead of: span=1,0,0,ccs,hdb3
this line is changed to
span=1,0,0,ccs,hdb3,crc4

So apparently with no reason FreePBX adds “crc4” to the configuration.
We suspect the file system.conf is taken to its default values but don´t know why and also would like to prevent it from doing this.

Thanks for any help!