My Spotmatic SPII does the same. I did have a theory that it was not the mirror, but actually the spring mechanism on the Auto setting of the lens, that returns the lens to open after exposure (but it was never cosnistent enough for me to prove it). When I was repairing an old Helios 44 lens, I tested the Auto mechanism a few times, and it was surprisingly similar in sound. Either way, it is normal....now get some film in it and enjoy...beautiful camera. Kal