update: took it back to who worked on it: they first adjusted something on the chain, not sure it was the masterlink, but that seemed to help a lot.
It was still doing it, even tho less noticeable, so I had them put on a new sunrace 6 speed freewheel to replace the old shimano 5 speed.

That seemed to help even more. I'm still getting a slight version of the noise when I'm in the second highest gear (big ring front, 2nd smallest back) which leads me to suspect that it may be something with the bottom bracket. But I'm trying to do my best to just ride the bike and stop obsessing over it.