"It was there regardless of gear. "
Are you are pedaling at the resonant frequency of the chain? It's unlikely but could it be the RPMs of your pedaling. Is there a peak in the vibrations, if you pedal faster or slower from the peak vibratiions does it get less and less?
2000 Montague CX, I do not recommend it, but still ride it.
Strida 3, I recommend it for rides < 10mi wo steep hills.
2006 Rowbike 720 Sport, I recommend it as an exercise bike.
1996 Birdy, Recommend.
Wieleder CARiBIKE (folding), decent frame.