Knocking noise in rear of 99 Honda Accord

My father has worked on cars his entire life and is stumped. My wife's 99 Honda Accord/ automatic transmission/ about 60K miles is making a clunking sound somewhere in the rear of the car. My father has taken a look under the car and can't find anything loose. He has jumped up and down on the car, pushed it side to side, and can't duplicate the noise. He's starting to wonder if something in the tank might be making the sound. Any ideas?
The car only makes the noise while moving and it doesn't seem to relate particularly to steering or braking. You hear it alot if you're driving over a bumpy road. Thank you.