system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5: why do car engines vibrate more when in park?
gpt
When your engine runs, it doesn't deliver nice constant power.  It's a series of captured explosions delivering sudden bursts of power, followed by a lag until the next burst.  This is more noticeable when the engine is running really slowly, since the explosions are further apart.  Also, the weight and resistance of the drivetrain helps to smooth out the explosions and make power delivery feel more smooth, but when the transmission is in neutral this extra weight and drag isn't present to help smooth things out.  Finally, engines are tuned to run best at a certain RPM, when they are running significantly faster or slower than this RPM, they can run more roughly.