If memory serves me, there's a TIS that talks about this. On the X5, there's supposed to be what looks like an air bubble on the lower corner (closer to the driver side - LHS) of the sensor. I have it on my X5. OTOH, my MCoupe doesn't have such air bubble

Why are you trying so hard to get rid of the air bubble? Is the rain sensor not working properly? As long as it's operating properly, I couldn't care less if there's a bubble or not.