Yup, will most likely be the coolant reservoir, they are prone to cracking after a while. Can get non OEM part for R650, quick 45 minute DIY job to replace, just beware cause there are 3 different types for the E46, take it off first then go buy the part so you make sure you get the right thing. The one hose clamp undernear the reservoir can be a b!tch to connect if its the same as mine.
Good luck!