A couple of steps in the staircase of our 1927 home have started to "thump" when stepped on. The thumping seems to come from the wall to the left of the step. Slight motion of the step accompanying the thump is noticeable, but not too much at all. I can't tell anything by watching the structure from below, in the crawlspace (is there a layer on top of the tread that I can't see for these old stairs?). I'm wondering how to diagnose, and whether to treat or ignore, the new sound.