If the whole knob is rattling (loose), turn it clockwise to tighten.
If it won't go all away around and it's not straight, unscrew it and place a pencil erasure (if you can still find one) on top of the shaft and screw the knob back on.
If the plastic insert is rattling (loose), pop it off and glue it back on.
At least those things worked on my miata ......
humfrz