I thought you said it kept on vibrating when you shifted into neutral? Maybe I'm confused between your post #8 & #10.

 

You're correct, it did continue to vibrate even when I put it into neutral. But the thing that confuses me is that it happens so randomly. It always happens at the same speed, but it happens very sporadically. Like it'll happen once and it won't come back for a couple of days, sometimes not for a week or more.