What kind of car? 2004 Accord EX Automatic or standard?

Mechanical vibrations if exist, are there to stay or get worse until you fix it. In any mechanical system, there is a natural frequency that vibration gets really bad and then you notice them until you pass that peak and they decay. An eample of this is wheel balancing.

You need to figure out what are the symptoms first. For example, you are going 25 MPH accelarating, you feel the vibration, now shift to neutral to take the transmission out of the equation.

You get the point, try different combination until you find the thing that is repetable every time.