It's almost certain to be item #17, the plastic bushing which locates the shift rod in the base of the gearshift lever. Every 8 years or so they get brittle and fall out. The lever will then buzz & rattle at highway speeds.

Good news is the bushing is less than $2. Bad news is it's a surprising pain in the neck to install. Use the search feature to read up on installation techniques.

Get two bushings, there is one at either end of the horizontal shift rod.