just wanted to report back on progress...
I had a sudden increase in the vibration last friday, so I pulled the car in the garage.. discovered while inspecting that a lot of road grit had accumulated on the inside of the wheels (unevenly) and that was throwing off the balance..
Changed all fluids along with cleaning the wheels and the bad vibration was gone, but that very subtle whomp whomp is still there, coming I think from the back.
I inspected all axles, drive shafts, etc and they appeared to be tight.
While up in the air I noticed that my right rear wheel seems to either be either poorly made or slightly bent, or there is an issue with the hub being out of round... there is a slight but noticable runout on the wheel, not sure yet if that is causing the issue but I'll try to switch wheels or have the current ones balanced and report back.