Cecilia, I also like the explanation of Macybaby. The Bernina was kept in a basement, so it may have cause the presence of moisture.
The pedal that made the same sound was also a metal one. It did it one time and never again.
I runned my Bernina yesterday for a few hours (without the motor cover to see anything bad happening) and I noticed it was doing it a lot in the beginning, but less in the end.
So, let's check out it turn... Good luck!