When I'm sitting at a light, or just idle, my car has a mild vibration. If there's a cup of water, it just ripples. However, when I'm driving it's smooth, and otherwise only has 85 thousand miles on it. Runs good. Any idea what this could be?

 

A vibration that you can feel in the car mostly likely means that a frame component is loose somewhere. The first step would be to try to pinpoint the location of the sound and then inspect and tighten any bolts and fasteners that you find in that area.

 

Sound?

 

A vibration is usually accompanied by a sound, no? It may be very subtle, but it will be your clue in helping you to solve the mystery.