Grating Tongsheng with power off

I believe this problem has been raised before but I haven’t seen any solution. After five or ten minutes with the power off I get a creaking sound on both sides on the down stroke. This steadily gets worse and develops into a loud grating with some vibration. It decreases when echo mode is switched on and disappears in tour mode. I have switched pedals and cranks with no improvement. It is important to me to be able to ride unassisted and others report the TSDZ2 to be silent with the power off. Is there a solution?