A loose or hard and shrunken sway bar mount will thump on the floor pan, too. Always at low speed when one wheel is deflected and not the other.
Take at look at those while you are under there. If they are bad, expect to replace the sway bar mounts up front, too.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!