I am trying to work out exactly which model you have of the E90, but my 330i has had this problem since I got it. Going to let motorplan sort it out soon since it is warming up.
Not sure if the symptoms are the same as yours, so will indicate what I am experiencing:
After a spirited but still reasonable drive on a warmer day I hear a squeaking or chirping sound from the brakes at low speeds. Note, this is when not using the brakes, simply driving along - applying brakes and the noise goes away.
Known issues with 330i and also 335i is that this noise is caused by the brake wear sensors rubbing against the disks. There are two of these sensors, on the front left and right rear callipers...