Convertible top motor noise - normal?

Hi, I am close to buying a low mileage Z4M roadster and this forum is very helpful. The convertible top motor was recently replaced. However, when I open or close the top, the car makes a rather loud buzzing noise. I can't tell whether the noise is electrical interference from a rear speaker or the motor or something else. Has anyone else experienced this? Is it normal? I had a e46 m3 convertible, but it didnt make this noise. Thanks