There’s lots of things it could be. The most common is an internal o ring somewhere. Get a bottle of UV dye and UV flashlight. Specifically look at the front of the engine, behind the water pump for a small weep hole. If you see a small dye stream it’s the common cross pipe front o ring leak. The back o ring leaks down the back of the engine.
There’s a kit to fix it now. It’s not too expensive.
https://xoutpost.com/bmw-sav-forums/...christmas.html
I used this one and it worked well. Prep, prep, and more prep! And follow directions exactly.
https://bimmerfix.com
There’s split pipe solutions as well, but they’re a little more work.