The bulb is not an overly great design and if you look closely you will see there are two wire strips as contacts. The holder is also a small shape and therefore the contact surface is extremely small.

The contacts corrode with moisture from condensation over a period of time and need to be either cleaned or bulb replaced. To do so requires removal of the light assembly in older X5's (about a 20min job)

The increased blinker rate is a warning that there is a bulb not working, and will continue whenever the contacts are not clean, as it is thinking the bulb is blown.

Cheers.