Belt too loose and it starts to squeal, belt too tight and the bearing in the alt can fail.
So check the tension.
Rule of thumb is when you can barely twist the belt by hand to a 90 degree angle, you're good.
Another thing is the 1150 GSA can have 2 different lengths of belts, did you get the correct one?
Compare to the old.
Up to a certain frame nr it is the short belt, from another frame nr it is the longer one, the numbers in between are suspicious and not even known by Herr BMW himself.
Paul.