I have an al1717 lcd monitor

That could be electrical interference, from your computer, or from the speakers, or from the AC power-adapter for your speakers, or from the AC power-adapter connected to an external hard-drive.

While the monitor is on, lift it away from everything, as far as the power and data cables will allow, to see if the "herring-bone" vanishes.

Also, try a different "data" cable between the monitor and your computer. Or, just disconnect the cable, switch it end-for-end, and reconnect it.