Strange pinging noise during calls
I have rebuilt my box and have run fxotune and messed around with my echo settings and echo is no longer really an issue - I have a bigger problem 
During a call I hear this pinging noise almost like a sonar bounce. I can hear it but the person at the other end can?t but if I’m talking during the noise they don’t hear what I was saying at that time. It doesn’t happen all the time and only maybe 2 times every call but its extremely annoying.
Anyone know what it is or more importantly how to get rid of the thing. I thought it might be an echo training tone or something or am I barking up the wrong tree.
Thanks