At night when my room is silent there's been a really annoying screeching noise, and I finally narrowed it down to my new EC2-B. When I put my ear around the sensor I can hear it clearly. Don't notice it during the day. When I move my cursor the whine stops for a couple seconds then returns.
Pretty sure my EC2-A and many other mice didn't have this. I made a post on Reddit and another user has reported the same problem too, and was suggested to post here.
Really though, that is weird as heck. I haven't noticed it with my ec2-b thus far.
Edit: So i cut everything off in my room just to see if i could hear what you posted about. Sure enough i heard it. I thought to myself "no way". Pick up the mouse and place my ear by the sensor, & hear the exact same annoying sound coming from the mouse. You aren't the only one.