Is it a whistling sound? It could be the motor itself. Mine makes some sound but really doesn't bother me that much. I have heard of others having this same issue as well but it is not a rattling sound as far as mine goes. Did you put some locktite on the screws as the install instructions suggested? If you did that it is not likely to be the source of your problem.

Can you test the shaker motor with the play field up? Give the machine a good shaking with it turned off and listen for any noise coming from the cabinet joints and leg mounts...

You also might want to make sure it is none of the wiring/connectors rattling on the inside of the cabinet. I know there are a bunch right in that area.

Loose play field plastic???

Is the game level side to side?

If that is the case you can get a ratchet strap put it around the cabinet and see if it stops...

Did you know the balls will rattle in the ball trough??? Maybe that is what you are hearing.

The one I had (have) is a deep rattle...it almost sounds like it hits a resonance that rattles something substantial. I can't track it either. I don't think its the back board or cable bundle. It sort of sounds on the left side but ... ????

I did have to put in some paper as filler to secure the backglass as it was rattling pretty good before I secured it a bit.