maybe something paranormal living there. very difficult to find out, but might be true because you said you confronted the wife, the wife claim not dragging the chair.

I know it might not make sense, but have heard of this type of stories before.

please dont be angry with me, just sharing because one of my friends unit have sound at night, at first thought piping contraction sound, but after checking, found got some dirty things disturbing. not easy to get rid of it so they moved out within short period. landlord accepted and did not forefeit his rental deposit, he believe most likely the landlord knows and just accepted.