I thought I had the same problem with my '95 850T wagon as all I could hear when I hit a bump was a ton of rattling. I never got in the back and had my wife drive the car so I could pinpoint the problem. In short, the rattle was not the tailgate, but the rear shock mounts. Both were shot and the racket was unbelievable. Up until that point of reality, I thought it was the tailgate. The broken mounts were smacking up against the rear cargo panels. It might be worth checking out.

This is my first 850, I've driven the RWD Volvos for 20 years. And, I must say, I enjoy the feel of this car. It fits like a glove and the power and ride is lovely. As for the shock mount design....

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'90 245 DL 297K 91 944T 226K