[Coco] Microbes - audio distortion from joystick movement?

[Jeremy Michea]

Was playing my microbes cart tonight and noticed that there would be 
some audio interference like a buzzing sound that would increase or 
decrease in intensity depending on the movement of the joystick as the 
ship zipped around the screen. Seemed to happen no matter if I used a 
Coco 1,2 or 3 and in the case of a coco 3 it would happen with both RF 
and composite. Could only test this one cart as it's all I have but I 
haven't tried it via drivewire to see if it happens with a virtual cart 
as well. Cleaned the cart contacts, which had no effect.

Any explanations as to what might be causing this?