That doesn't sound like electronic interference (usually gets more annoying and matches speed with the engine).

Can you do a test? Remove one camera from the protective case and put it in a soft padded bag or your jacket pocket, leave the other camera as it is. Turn them both on and go for a similar test ride while recording. See if the two are different.

To me, that sounds like vibration rattling the camera. Both cameras are mounted to the bike which transfers the same vibration. This is why I want you to isolate one camera to see if there is a difference. If it's really electronic interference, the noise will be there regardless (and would affect the camera at any RPM range). If it's not present on the padded camera, you have a vibration issue to figure out.