If your Mustang trunk or hatchback rattles it could be that the weatherstrip has compressed to a point to where it isn't holding firm against the trunk lid or hatch. Replace your Mustang trunk or Mustang hatchback weatherstrip with one of Late Model Restoration's quality reproduction pieces.
These Mustang Trunk & Hatch Weatherstripping fit: 1979, 1980, 1981, 1982, 1983, 1984, 1985, 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, and 2009 Mustangs.