So, the other day when it was snowy and icky out I had to drive my car.
It's normally a turn-the-key-start-right-up car at this time but on Monday
it kind of turned over a few times and then belched a bit smoke cloud out the back. Strange for this car that hasn't smoked a day in three years.
Then it was idleing REALLY loud and a bit rocky, for the first time I could FEEL the idle.
When I put it into reverse the jackhammer sound started and it rocked a bit more. Not a high pitched tinny sound, but a low grumbling jackhammer sound.
When it's in drive, Neutral or park, the idle is almost the same as normal. Just ever so slightly rougher. Just.

When I go to turn off the key the car hammers for about half a second or about 3-4 times before it dies.

I haven't checked under the hood, but when I do what should I be looking for?
Thanks!

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Ginny in Denver-ish
78 300SD, 265K (mine)