I recently got a nice Canon 7 and I am loving it. But there is one thing about the shutter when I fire it. From speeds of 1/1000 to 1/60 they sound fine, but when I use the lower speeds 1/30-1 sec, its like there is an additional sound like a spring bouncing. Is this normal? I have developed 2 rolls of test films that I have used with it and they were all perfect, no pinholes etc., but I am worried about this weird sound. Any thoughts?