HI Andy and AlanP
The noise is caused by the motor coils resonating due to the ESC switching frequency falling within the audible range.
Used to be quite common a few years back and some manufacturers provided a means to alter their frequency to help users.
If you still hear a noise at neutral motor position it is possible you need to adjust your trim as there should be a deadband where almost no current is flowing, and no noise.
If you supply more details as to the motor ESC and radio gear it may be easier to be more specific. is the motor controlled from the tx stick in the centre position with forward and reverse control?
Really more of an annoyance than a fault, but if you can adjust your setup then at least the noise will cease when the motor is stopped.
Dave 😀

This is one for Dave M when he wakes up, 😁 but he will probably want a bit more information.
Like, is the noise coming from the motor or the ESC, is it a brushed or brushless set up, make of motor and ESC, and that's just for starters.

Alan