There are only four things that can go wrong with the autolevel system:
The pressure "bulbs" can go bad, leading to a locked rear suspension -- the struts cannot move because there is no gas space to compress -- the membrane bursts and the cylinders fill with hydraulic fluid. You will know if this happens because the tires are the only spring in the suspension and the rear end will beat you to death on bumps -- unmistakable bouncing. Not a bad fix, just a little messy. A rough driveway can just about shake your liver out!
The seals in the struts can go bad, in which case the car squats in the back and looks like the bumper is about to drag on the road. Not too hard to fix, but expensive, about $650 a side.
The actuating valve on the rear axle can go bad, causing the rear end to run up and down all the time, possibly in concert with road roughness -- or fail to actuate and refuse to level the car. The valve needs to be calibrated by someone who knows what they are doing to prevent excessive leveling and/or hunting.
And last, the pump can go, in which case the rear end usually sags, too. On diesels, the pump is on the engine, on gas models it is on the back of the power steering pump. It can be rebuilt separately. It is very expensive to replace.
I did the gas chambers on my 300TE -- the hardest part was getting the hydraulic lines back in -- they are short and stiff, so one has to get the correct end started first, and it took a couple tries.
Wagons in general are noiser that sedans -- all the rear end noise that usually gets muffled in the trunk space gets amplified by the rear floor instead. You might want to check to make sure the spare tire and tool kit are tied down properly, too -- if there is something loose, it can "dance" on the floor when you go over a rough spot, and make the car "roar". So can a bad exhause system, pounding on the floor.
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!