Strange buzzing noise from my electric bike motor only when climbing hills

I've got a mid-drive electric bike with a Bosch Performance Line motor, and everything was smooth until recently. Now, whenever I hit a steep hill and push the motor hard, there's this unusual buzzing sound coming from the motor area. It's not a grinding or scraping noise, more like a high-pitched vibration, and it stops as soon as I ease off the throttle or get to flatter ground. I’ve checked the chainring and pedals for any looseness, cleaned and lubricated the chain, and even updated the motor firmware through the Bosch app, but the noise persists. I'm worried it might be a sign of internal wear or some kind of bearing issue, but I haven’t seen any leaks or obvious damage. Has anyone experienced a similar buzzing under load on hills? Could it be something as simple as a loose cover or a misaligned sensor? Also, how urgent is it to get this checked out before it potentially causes bigger damage to the motor?