What causes the motor of a SC2 to vibrate at idle?

If the engine is running smoothly, vibrations are mainly caused by bad engine mounts. Top engine mount is to prevent excessive torsional movenments only and the main engine and transmission mounts are the ones taking all the weight and vibrations.
The other mounts should be checked.

Just to add to this one the most common mount to fail on that car is the upper motor mount. Especaily with the low mileage that's the only one I. Would that would have failed. If you look at the part of the mount facing the coolant tank you can see if its colasped or torn. If its the style mounting iam thinking of that would be on that car. There are two styles if you snap a pic of it I can help you a lot more on how to tell if its bad unless its really bad and then you just know its bad. Also that twin cam. Isn't exactly like the 1995 twim cam in 1999 they narrowed the head which gave it much smaller liffters. Also elimated the top timing chain guide which makes it a lot harder to tell when it needs a new timing chain by sound. Also starting in 2000 they chamged the intake manifolds on the twiln cams to. Plastic.1996 started plastic valve covers on the twin cams and very early production 1994 twin. Cams had plastic valve covers.

Best place to view the gap is the side facing the coolant reservoir bottle. 1 Mount you can look at it and see if it's frowning if it's frowning the mounts bad. Not mount use a 5 millimeter Allen wrench to see if you can put it in a square hole facing the reservoir. The other style mount that doesnt frown you use a ink pen disposable see if you can fit it between the part that goes on the front cover and between the mount itself. Like a bic that comes a 10 pack of pens.

If you get that car and end up replacing the mount don't replace it with another Mount that is the 1 that frowns style when it goes bad