1998 BMW 318 Shakes when idling in Drive position.

I just bought a 1998 318i. Its in really good shape, but still a couple of things that need attention. One of them is, when the car is at a stand still in the drive position, it shakes. Its not really bad, its just enough to annoy you. I had an older 1986 318i once that had the same problem and it winded up being a bad engine mount. That was my first though, but what other possibilities could cause the car to shake in this situation?