In the space of only a few weeks and 800 miles of taking delivery of my brand new Z4 M40i this problem has occurred four times. When it happens it sounds absolutely horrendous. The noise is a very loud high pitched squeal and it sounds like the wheel is just about to fall off. I have also noticed that it usually only happens when the car is slowing down close to stopping. I have a theory for this, which I will explain later.
There is no jack in the car to lift up the wheel to see where the stone is trapped and you either need to try to free it by driving, sometimes several miles and causing damage by scoring the disc rotor, or call out the BMW breakdown service.
In many years of motoring in BMW's and several other brands of cars this is the first time I have ever experienced such a problem. I believe that there is a fundamental design flaw in the structure of the M40i brakes as I will explain below.
Most modern cars have disc brakes and they can develop a lot of heat so the manufacturers incorporate a metal shield right behind the brake rotor to prevent unwanted heat transfer to brake pipes, ball joints and other heat sensitive parts of the car. The shield is also there to prevent stones, debris, water and dirt getting into the brake assembly. Normally, this shield is bent over the top of the rotor to close off the gap where debris and stones can fall into the brake assembly. On the M40i both the front and rear metal shields do not bent over the rotors. In fact the front ones actually bend away from the rotor creating an even bigger gap for stones and debris to fall into the brake assembly. The gap is large enough for small stones and debris to pass through but anything larger than about 1cm (1/2”) in size is likely to get trapped, causing the problem that I am describing in this review.
In the UK, many of the roads are covered in stones washed up out of pot holes and decaying road surfaces so it is very difficult to avoid getting stones thrown up into the wheel arches of our cars. I have also noticed that the Z4 M40i is very prone to throwing up stones and this might have something to do with the 19" Michelin Pilot Super Sport Performance tyres. These two things are exacerbating the problem.
On examining the location of the brake rotors on the M40i it is difficult to understand how the stones are getting into the rotors as the wheel rims are extremely wide and the rotor assembly is near the centre of the rim. Also, the rotors are quite large in diameter so the space between the top of the rotor and wheel rim is quite small, making it very difficult for stones or debris to get thrown directly into the brake assembly.
My theory is that some stones are being thrown onto the wheel rims and the centrifugal force of the wheels rotating causes the stones to stay on the wheel rim until the car slows down at which point they fall down into the gap between the brake rotor and metal shield. Any stone or debris larger than about 1cm is likely to get trapped, particularly since the design of the metal shield does not prevent the stones and debris getting into the brake assembly as it should.
I believe that the problem outlined above is a design flaw in the Z4 M40i brakes and BMW should stand up and take responsibility and offer an immediate remedy for the affected cars. I have already requested my Dealer to take this issue up with BMW.
I would like to hear if any other Z4 G29 owners have had similar problems with their car.
Uh oh....E89 wheel cracks.....G29 stone traps....and so it goes. 
My ///M brakes occasionally do that, but they have always cleared them selves within a few miles. Hope it gets sorted quickly.
many a time we have had to reject half a lines worth of engines (approx 150) because of silly errors like this