Three months ago, my husband and I purchased a '98 Z3 2.8 with 33k miles. After the engine is fully warmed up, it makes a noticable squealing sound--similar to a belt that is slipping slightly. Our local BMW dealer replaced both belts but the noise persisted. A second visit to the dealer resulted in an explaination that the noise was due to a "vacuum shift" and it was "normal engine noise". Next, we took it to an independent garage where they confirmed that the noise was not normal but, other than saying it was loudest around the clutch assembly, were unable to make a diagnosis. Anyone have any ideas?