rhythmic vibration

This is just something to try to narrow down which end of the car it's coming from. Disconnect all the belts at the front of the engine. Start it up and idle it and also rev it to different rpm's. If no change then you can eliminate any pulleys or the fan or fan clutch.

One thing you can try only if you have a place to take the car and try it is to try going down a long steep hill, put the car in neutral and shut off the engine and see what happens. That takes the engine and trans pretty much out of it and then you can focus on the driveline. If it goes away you know it isn't the drive line. If no change then look to the driveline. I had a similar ordeal with my Dart and mine ended up being a out of round driveshaft. It started out small and got progressively worse. I had to have a new shaft made and that took care of it for me. At least it will give you and area to look to by eliminating engine and trans.