I would check engine compression - if you have one cylinder that isn't doing the same work as the others, you will get some vibration - if all the cylinders are within 15 percent of each other, then I would suspect a broken piston skirt or a bent connecting rod - could also be the torque converter in your tranny. If I'm not mistaken your accord engine has a balance shaft in it, which could be out of time as well causing vibration. All this is assuming that you've already tuned up the engine (plugs,wires, cap rotor, etc.) I would also check your egr valve, it might be stuck open - CEL ?