Sounds (literally!) like you have a vacuum leak in the actuator. If you hear it hissing, it's time to pull the door apart and fix it. P.S. Sooner or later the closing assist pump in the trunk is going to log an error and stop trying to close the door. Don't worry. After you fix the leak , pull fuse #9 in the trunk's fuse panel to reset the pump.

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"It's not about how fast you can go, but how well you can go fast."

Bob in Richmond

'97 S320 (LWB), Ruby Red Metallic, 73k miles
'97 S420V, Smoke Silver Metallic, 155k miles