I live in a 100 year old home that was rehabbed in 2008. Recently(past 6 months) three of the 5 bedrooms on the second floor feel an oscillation(back and forth movement) with no detectable cause. it happens irregularly(when people are moving about, no one but me at home, no one source seems to cause it, it just happens). I have no been able to find any similar problems online. There is slight floor bounce, but the floors are solid.
What could be cause of this, and how can I remedy it? it needs to be fixed because it is disturbing. What can be reinforced/braced to prevent this?