Click noise on E1 TE121

We have a TE121 for external communications withing our freepbx system (asterisknow). It has the hardware echo canceller. (Wildcard TE121 (VPMADT032))
All internal calls work perfectly. External calls are also working but remote speaker can hear a glitch each second.
Hehe, i’ll try to describe de noise. It is almost like a heartbeat but the first tac is almost inaudible.

Searching digium documentation for this issue we found this answer on the FAQ:

Why am I getting a clicking noise?
If a clicking noise is present when dialing through an FXO or when getting dialtone from an FXS, this is cause by echotraining. Turn it off to get rid of the clicking. The click is necessary for the echotraining.

As read in the forums, we tried to disable the echotraining (using include). The chan_dahdi is configured this way:
; span_1

But it didn’t work.

We also tried to remove the hardware echo canceller but we still hear de glitch (so, it seems the echocanceller has nothing to do on this)

We also looked for system errors related to interrupts…etc. but everything seems to be ok. The voice and sound quality in fact is absolutely great. The problem is that glitch wich is very anoying if there is a silence (it also sounds on MOH or conversations but not really loud)
MOH or voice is not affected by the glitch. I mean, there is not any interruption or corruption in the voice, both sound simultaneously.

any hint about this??

Hi, we solved it.
The problem was on the sync source configured for the span in the /etc/dahdi/system.conf
it was set to 0 when the correct value was 1 (sync source on the far end)

hope this will help someone in the future.