Slight rhythmic vibration at idle

I have noticed that my car has a slight rhythmic vibration while at idle. I have had it for a while (don't really remember NOT having it), but recently had a ton of work done on my car, including: new spark plug wires, O2 sensors, intake manifold gasket replacement, and a new catalytic converter. I thought this would take care of it, but it hasn't. It seems to be coming from under the center of the car underneath the floor boards, and you can feel it in the steering wheel if you think about it. I did recently get new tires, which I feel is making the engine work harder in general (still breaking them in). I didn't think to bring up this vibration to any of the mechanics, but none of them brought it up, either. And they have been pretty good about letting me know about any observations that they have about the car since they have spent so much time with it. Any ideas as to what this could be? There is also a small click noise that you can hear when the radio is off and the windows are down when shifting from Park to Reverse, to Drive. However, the loudness of the click depends on how much driving the car has or hasn't been doing lately.