Could easily be the parking brake cable but could be just about anything else. Is your spare tire securely fastened? That will make a racket if loose. Any tools/toolkit in the trunk? Other than that, get down and inspect every rubber bushing back there and replace any that look worn, 198k is a long time to be on stock rubber.

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TC
Current stable:
- 2004 Mazda RALLYWANKEL
- 2007 Saturn sky redline
- 2004 Explorer...under surgery.

Past: 135i, GTI, 300E, 300SD, 300SD, Stealth