Hissing usually means that the gas has leaked into the oil and is making noise.

Eventually the ride quality will start to suffer, and it will need replacement, but mine have 200,000 on them and work well, so I'd only replace it if the car starts to bounce or the wheel tramps.

Peter

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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!