thanks for your help!
my singer 31 has always done this and i somehow got the idea it was normal
in my singer 31, the needle bar connecting link (43997) is giving the tension releasing lever (43944) a little bump each time the needle goes down. the machine still works but it's enough of a bump to see the tension discs release a little bit for every stitch.
i wonder what i have to adjust to stop this from happening. it looks like the spring (12329) is pushing the tension release lever out too far and into the path of the needle bar connecting link.