Hi David,
This is a strange one, first of all I’d take the wheel off and see if there is any debris or signs of slight warping. It’s possible as the wheel goes faster the noise is being masked.
When you have the wheel off check the wheel, hub & brake pads also.
If you can’t see anything obvious I’d swap the wheels and see if it persists. If no then it’s the wheel otherwise it’s probably a trip to your local garage.
