Accord squeak on bumpy patches
 

My 990 Accord LX, 135k, has in the past few months developed a slight
high-pitched squeak when I drive over especially bumpy surfaces. It's
not loud, not a squeal -- it's almost just like an old chair might make
or something, clearly timed with the bounce of the car. Sounds like it's
from the back of the car, but it can be hard to tell.

Any ideas what's going on? Should I be lubricating something that
wouldn't ordinarily get addressed?