I had the same cowbell clang on my '03 LT. Dealer said they had an advisory on it from BMW. I asked about an appointment to have it replaced. I was told that the advisory was to alert the dealers to the noise problem but no warranty or help from BMW. They quoted me $475.00 for the new rotor assembly and $75.00 labor to install.
I took the bike into my shop and dismounted the rear wheel, examined the rear rotor and found it to be in near new condition after 18,000 miles. I decided then and there that I was NOT going to throw away a perfectly good rotor. I measured the "float" distance between the rotor and the hub and found it to be 19/1000th of an inch. Below is the technique I used to correct the "BMW Cowbell Clang".
I now have over 500 miles on my reworked back rotor and I have no more Cling, Clang. It doesn't help the little screetching from the brake pads that comes and goes, but everyone seems to think it is the compound of the OEM brake pads. I am waiting for my time for pad change, which will not be OEM, to see if that can be solved also.
The re-work for the rotor is fairly straight forward. Remove the rear wheel, remove the two bolts securing the rear caliper and suspend the caliper out of the way to remove the two bolts securing the brake rotor hub from the final drive. Once you have the hub rotor assembly on the bench and cleaned up, you need to gently wedge between the rotor and the hub with something like knife blades or wood chisels in at least three equidistant places around the hub. This will hold the "floating disk" at the far side from the hub on the float pins. Then, using a metal shim feeler gauge, measure the distance between the hub and the rotor. Mine was 19 thousands of an inch.
I obtained some 19 thousands shimstock and cut 1/2" x 1/2" square peices. Then, on one edge of each shim , bent a 1/8" inch x 1/2" 90 degree bend.
These angle bent shims are now gently tapped between the hub and the rotor from the outside, centered between each set of float pins. The little 1/8" flange on the shim stops it from sliding through by stopping it at the edge of the hub. As I recall, I needed eight shims so there was one centered between each pair of float pins.
I then set the hub/rotor assembly in my bench vise, gently clamping on the rotor with one of the shim flanges now setting firmly against the vise jaw. I then used a utility knife to lift and bend another flange on the shim on the inside of the rotor. After the flange is bent 90 degrees on the inside, I used a small pin punch to firmly swege the newly bent flange tightly against the rotor's arched cutout. Then rotating the hub rotor in the vise, I proceeded to do the same with all the shim spacers. When they are all sweged tightly, put a drop of super glue on the inside flange bend on each shim, allowing the glue to run between the rotor and shim.
This basically turns the "floating disk" into a non-floating disk and eliminates the "clang clang" tune. My first attempt did not resolve the problem as the shim stock I had was 18 thousands, and left an almost imperceptible amount of float on the disk. The clang was greatly reduced, but not completely gone. You need a shim that will eliminate all float to be successful. Total cost of repair $2.50.
