I have an older Trek 930 with a 250 watt Bionx system which is only a year old. I unpacked the bike after winter storage in the basement and I've been riding to/from work for the past 2 months. Starting this spring, I am getting a grinding feeling most of the time when the motor kicks in. Because it seems to come and go, I tried to figure out what the pattern is. It turns out, if the front wheel of the bike is turned even slightly to the left, the grinding happens. If I am steering right, it goes away. If I zig zag, it comes and goes as I go left and right.
I'm pretty sure the problem has something to do with the Bionx system since it feels like it is coming from the back and disappears if I turn the system off.
Is it possible the regen is kicking in? The most common problem I have had with my Bionix is the cabling. Occasionally a connection will come loose and cause inconsistent behavior. In this case I'd pay close attention to the magnetic switch on the hand brake that engages the regeneration. That could easily become sensitive to handle-bar movements if loose or misadjusted.
I had this on my home built electric bike. If I turn left I get a grind (feels like rubbing sand on paper). It sound like your left bearing has become dirty. I don't know this bionx system but if you look at your front wheel, in the middle you will find bearings on the left and right of the hub (motor or not). You can try adding a good amount of sewing machine oil to lube and flush out any dirt. But if the ball bearings inside the bearing are gone then you will need to replace the whole bearing. Bearings usually only cost about $9. Mine did. But I had done 5800km on my electric bicycle though. So I just bought 2 new bearings. If you are unsure take it to a bike shop.
Ok my bike had a front hub motor. I just read ur post again and googled bionx and it is a rear hub motor.
I also see you only get the problem WHEN the motor is turned ON. This nearly eliminates bearings (but not totally). Points to maybe mistimed hall sensors but ONLY when you turn left so that eliminates hall sensors. So basically it only happens when torque is applied and turning left. Hmmmm. Maybe a bearing works under pedal power but can't handle the power from the motor anymore? Hopefully that's all it is.
Another possible cause is that one side of the magnets inside the hub motor touch the rotor when power is applied. This will also give a grind feel only when power is applied. It will only happen for a few weeks or months then the hub will stop working. I've had this happen too.