FxLMS filter in feedback ANC application

I'm trying to use the FxLMS filter (running on ADAU1467) in a feedback ANC setup i.e. the sensing microphone feeds both the reference input and error input. Setting the secondary path filter using training mode seems to yield something that is very unstable, once the noise signal exceeds a certain level the system goes into low frequency oscillation. Does anyone have any ideas how to improve this please?