Bafang Max M400 strange noise - is the motor dying?

[Letterman]

My 250/350W Bafang M400 Max drive has started making a strange whirring noise (not the electrical hum that I’m used to). I first thought it was the chain, but it isn’t. The noise only appears when the motor has warmed up. When the motor is turned off and I’m pedaling without assistance, there is no noise.

Below is a link to an audio recording I’ve made of the noise. Please excuse the background noises and wind I did the best I could):

https://sndup.net/7d7p

Any idea what this noise could be?

Thank you for your help.