Sizzling problem

Hi everybody

I need help concerning a sizzling problem with Doro Matra analog phones and asterisk. Those phones are connected to grandstream 4024 gateway. I use 3 gateway. On the first one, there is no problem so I compared its configuration with the third one witch is causing sizzling problems (after having upgraded its firmware). I’ve tried to clone the configuration where possible but the problem is still present. What could cause this please?

Thanx for having read.

have you tried moving the phones from gateway 3 to gateway 1 and see if the noise moves with them?

This is the solution I chose and it succeeded. But I wanted to understand the cause to avoid it the next time.