TDM400P Beep every second

I have a TDM400P with only one FXO module running Zaptel 1.4.6 with trixbox 2.4.2, and whenever I call my zap trunk, I hear a light beep in the background every few seconds. It is louder when the music on hold is playing.

I have tried various zapata.conf files and various builds of zaptel drivers to no avail. Does anyone know how to get rid of this beep?

Thank you in advance!