Like asking for economic advice, imo...
it is one of
the wonderments on X5s, and the "solutions" are numerous and often anecdotal, imo.
A quick look on the Search Tab found this list of threads, and there are pages on the topic
depending on what terms you use for "search".
Good luck on the diagnosis and I hope you don't "part it" to death in the fix effort.
GL,mD
Shudder While Braking:
http://www.xoutpost.com/search.php?searchid=1506209