Screeching noise from 97 Z3 rear end or AT

I have a 97 Z3, 1.9L eng, 48000 miles. When the Automatic trans shifts from 1st to 2nd and from 2nd to 3rd a screech noise is heard. I changed the rear end and trans fluids (and filter) but still hear the noise. Performance is excellent and BMW dealerships said they never heard of the issue. Any ideas?