I had an '02 4.4 that lost reverse one day. Did a bunch of research and it was caused by the small 'ball' check valve that disintegrated and then chewed up the rest of the transmission. All the friction plates were toast. Luckily i had extended warranty and BMW replaced tranny for $50 deductible! $7K job! If you are lucky you will be able to just replace the valve body on the tranny.