Check the fluid level in the hydraulic reservior beside the ABS unit. I suspect you are low, and pumping air through the struts in the back, making them groan. Fill and look for leaks.
Also, check to make sure the struts allow good suspention movement. If they are VERY stiff and bouncy, replace the accumulators. They are under the shelf between the rear seat and third seat, under the carpet. Hydraulic lines attach outside the car.
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!