So I talked to the stealer today and I was told that to remove the wheels and clean the brakes thoroughly (as advised by them last time I went there), would be a paid service and BMW does not pay the dealer for 'environmental damages'. I was quoted 2hrs of labor!
I mean...com'mon, what do these guys think...? My car is just 1.5yrs old and I have to pay for something that is not right with the car itself! Can anyone suggest what could be the problem? It is definitely not the brake pad sensor since they checked it last time. The sound comes only when the brakes are wet (in rain or after a car wash).

Another questions, do I need to get the wheel alignment done, in case all four brakes are serviced?