RCI 2970 squeels until you unplug the external...

Question

I have a RCI 2970 and it will squeel until you unplug and then plug the external speaker back in... I key it up and then it will last about 15seconds and then it wont squeel again until i key up again... what could be doing this... I have tried different speakers and different grounds but nothing will work.. It seems like when it warms up it doesnt do it as often