Suggestions for fixing this bass rattle?

[cuzzin]

I am using a shelf I bought from IKEA to hold my media equipment. The only problem with the shelf was that it had no openings in back that wires and plugs could go through. So I had to go and cut the openings myself. Only problem is this appears to have weakened the board behind the center channel and now, when there is a high amount of bass being output from that channel, the board behind it rattles. Here is an example of what I'm talking about (this scene from Jason Goes to Hell is when I first noticed it):

YouTube

As you can see, when I apply force to the back board with my hand, the rattle stops. Any suggestions on a more permanent solution?