Sorry to give you the bad news, you can screw around all you want trying other things, but it's the trailing arms. They wear out at about 80K. You can look at them and not see anything wrong, but the plastic bushing wears enough that when there is a lot of load put on them, like going over a bump, they'll make a knocking or banging noise.
In my case, they were making enough noise that it was difficult to have a conversation in the car. But after replacing them, then I could hear all the other suspension parts that were shot, and I ended up replacing everything else. My car had 145K on it, though.
I was able to replace them on ramps, but it's really tight. I used a long enough extension and a long breaker bar to get them loose, and then made sure they had the correct torque when tightening them. If you don't correctly torque them, they'll make noise because they aren't tight enough. I didn't get an alignment, and the car drove fine.
The rear trailing arms seem to be the first thing in the suspension that wears out, so you have a lot more parts to look forward to replacing in the near future. Welcome to Porsche ownership!
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