I have the same issue as the original poster. Model X 60D. Has anyone gotten this issue resolved? So far I learned it has nothing to do with bluetooth, my phone, or electrical noise. The problem is related to the microphone itself, how bad it is at picking up directional sounds (from the driver) vs. all sounds (this is called polar pattern/directionality), the direction in which it points (could this make a difference from car to car?), how it is attached to the car (e.g. is it sufficiently insulated from the car's vibrations), and whether there's an obstruction between the microphone and the driver (forgotten protective film??).
I have driven many (rental) cars and my MX has by far the worst voice pickup level and quality of all.