The parking brakes on the rear wheels can get rusty, because the drum brake does not self-clean itself as efficiently disc brake. This happened to my 400E, I just removed the rear discs and exposed the parking brakes, gave it a through spray with brake cleaner, and greased the joints with brake paste, then readjusted it, the noise was gone since then.
99 BMW 540i 6-speed 110K Km
03 SAAB 9-5 wagon 80K Km
92 400E (Sold) 245K km
Still missing the days with the Benz, it kept me busy.