It's difficult to tell without listening, but I suspect you might have a car stereo system with a large and somewhat overblown bottom end, and a mix that's been created on nearfield loudspeakers with more modest lows. Thus you're not hearing the offending frequencies until you get in the car
Alternatively, a little multiband compression might tame excess low-end dynamics.
Yet another alternative is that it's just your car stereo system that rattles at some frequencies, and nothing to do with your mix
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