does the knob only rattle/vib, or is it the whole shifter assembly (i.e. does the handle/arm rattle too)?
did the shift knob screw on nice and tight?
im just confused, as usual. the shifter transfers any drivetrain vibes, so it is normal. the stock shifter had the rubber isolator in the handle for these vibes.
now if it has gotten worse lately, it could be your balancer, DS, mounts, etc, depending upon when the vibes get worse.
good luck.