osmo-nitb + osmo-sip-connector: background noise on peer to peer calls

Hi, I am using osmo-nitb + osmo-sip-connector and nanoBTS to establish a per to peer call. I am using: osmocom-nitb 0.15.1.20161024 osmo-sip-connector 1.20161024 While in call, I hear an annoying background noise while not talking. The noise disappears when someone speaks. Any idea what might be causing this and how this can be fixed? Thanks, Stefan